Mass Shooters/Occupations

Eric Harris & Dylan Klebold: Blackjack's Pizza workers
Aaron Alexis: Civilian IT contractor, U.S Navy sailor
Randy Stair: Weis Supermarket, worked at a hospital for a short time
Adam Lanza: Worked at a computer store for a short time
James Oliver Huberty: Welder, security guard, worked at a funeral home
David Ali Sonboly: Newspaper delivery boy
Pekka Eric Auvinen: A friend told the media that he and Pekka worked at a local farm during the summers (probably where Pekka got the money for his gun)
George Hennard: U.S Navy, Marine Merchant
Nidal Malik Hasan: U.S Army major
Ivan Lopez: Army Specialist
Muhammad Youssef Abdulazeez: Engineer
Omar Mateen: Security Guard
Stephan Paddock: Accountant, Real Estate Investor
Nikolas Cruz: Junior Reserve Officers' Training Corps member, used to work at Dollar Tree
Chris Harper Mercer: Was in the U.S Army but was discharged 5 weeks after employment
Elliot Rodger: Worked at a construction business during his late teens
Marc Lepine: Worked part-time at the hospital his mother worked at, where he served food and did some custodial work
Thomas Hamilton: Former shopkeeper, Boys Scout's leader
Michael Kenneth McLendon: Worked in the police department for a brief period of time, Reliable Products warehouse, Pilgrim Foods, Kelley Foods(Sausage factory)
Heinz Schmidt: Teacher
Karel Charva: Taxicab Driver, Security Guard, claimed to be a psychologist
Anthony Barbaro: Worked at a local fast food restaurant with his mother
Charles Carl Roberts IV: Milk Truck driver
Mohammad Ahman al-Nazari: Mujahideen soldier, School Bus driver
Matti Saari: Was in the Finnish Armed forces for a month
Steven Kazmierczack: Was enlisted in the U.S Army

Michael Ryan: Antique dealer and Labourer
Richard Durn: Former Middle School Teacher
Charles Whitman: Served in US Marine Core
Anders Breivik: Entrepreneur, Trader, Salesperson
Patrick Purdy: Used to be a male prostitute and worked at several Machine Shops
Baruch Goldstein: Physician
Derrick Bird: Used to work at a nuclear reprocessing plant and was a Taxi driver

TheOne99 wrote:

Steven Kazmierczack: Was enlisted in the U.S Army      

He also worked at a prison for a brief time, but quit after getting to work late once. I'm also pretty sure he had at least a couple of retail/low level jobs.

Jennifer San Marco: Postal Worker
Robert Sartin: Clerk
Ľubomír Harman: Served in Military, employed at a Wood Industry, and was a boilerman
James Holmes: Counsellor at a Summer Camp
Howard Unruh: Served in US Army and was a sheet-metal worker

Kimveer Gill: 1 Month in Military; Various low-end jobs

Jiverly Wong: Delivery man, worked at a local Shop-Vac vacuum cleaner plant
George Sodini: Systems analyst

Arcan Cetin: Bagger at the Commissary in Naval Air Station Whidbey Island
Ibrahim Shkupolli: Grocer
Robert Hawkins: Worked at McDonald's but was fired

Scott Paul Beierle: Served in US Army, former English and Social Studies teacher
John Allen Muhammad: Served in US Army
Farda Gadirov: Worked at his uncle's business

Carl Robert Brown: Joined the Navy, teacher

Also, Anders Breivik had a farm.

MAB1996 wrote:

Also, Anders Breivik had a farm.

Yeah he rented a farm and registered himself as a farmer so that he could legally buy ammoinium nitrate fertilizer.

Alvaro Castillo worked for a soccer supply company after he left the National Guard

Stephan Balliet: Broadcast Technician

Alec Minassian: worked software developer intern, was a college student

If nobody else's already posted it, Dylann Roof worked for a lawn maintenance/landscaping company.

Euler Grandolpho: Systems analyst

Mehdi Nemmouche: Jailer and torturer.

N/A

Gabriel Wortman: Denturist
Sergey Semidovskiy: Police officer
Amy Bishop: Professor at University of Alabama

Zephen Xavier: Trainee prison guard at Avon Park Correctional Institution

Robert Poulin:Worker at a pizza shop & newspaper boy.

James Huberty: Embalmer and welder

William Atchison: Clerk at gas station

Dmitry Vinogradov: Lawyer at Rigla pharmaceutisical company

Alexandre Bissonnette: Former Cadet

Erik Salvador Ayala: Worked at Burger King
Wellington de Oliveira Menezes: Worked as a car washer, he then worked in a factory as a general services assistant, got promoted to a warehouse auxiliar, then he left his job in August 2010
Terry Ratzmann: Computer technician
Alvin Lee King III: Worked as a high school math teacher
Spencer Hight: Did contract work in Texas Instruments, until he lost his job, in 2015
Sulejman Talović: Started working at Aramark Uniform Services, eight weeks before the shooting.

Killisaki wrote:

Erik Salvador Ayala: Worked at Burger King
Wellington de Oliveira Menezes: Worked as a car washer, he then worked in a factory as a general services assistant, got promoted to a warehouse auxiliar, then he left his job in August 2010
Terry Ratzmann: Computer technician
Alvin Lee King III: Worked as a high school math teacher
Spencer Hight: Did contract work in Texas Instruments, until he lost his job, in 2015
Sulejman Talović: Started working at Aramark Uniform Services, eight weeks before the shooting.

I'm glad you mentioned Spencer Hight. He is a very underrated shooter, probably because he was a domestic one.

Martin Bryant - university professor.

John Russel Houser was a bar owner for a few years.

Jakob Wagner was a teacher’s aide at the High School where he later did his shooting at the prom.

Jennifer San Marco also worked as a lunch lady in a Highschool and a prison security guard.

Alek Minassian was also in the military for a short period of time

Bastian Bosse worked on a hardware store after he graduated from school.

From the book “Der Riss in der Tafel - Amoklauf und schwere Gewalt in der Schule”: "Er hatte die Schule 2006 mit der Fachhochschulreife abgeschlossen und arbeitete seither als Aushilfskraft in einem Baumarkt".
rough translation: “He had graduated from school in 2006 with a technical college entrance qualification and had since worked as a temporary employee in a hardware store”

Apparently there’re a shit ton of german books about school shootings, if anyone feels like checking them out.

Devin Patrick Kelley: Worked in the Air Force but was dishonorably discharged. He was a security guard at Summit Vacation and RV Resort at the time of the Sutherland Springs shooting.

Fabiano Kipper Mai (daycare stabber) worked at a clothing factory.

Saari was in the army for a short time, Kazmierczak also was in the army and had a job in a jail as a correction officer.

Jacob Tyler Roberts: worked at the Portland restaurant "Big Bertha's," a late night eatery. He was allegedly "loved by his employer." He also wanted to go into the military, a plan that was scrapped after be broke his foot in a bike accident.

Kip kinkel worked in the prison library and is now an electrician there.
If I remember correctly auvinen work for a time on a farm.
Brevik sold fake diplomas online

Karl Pierson worked at McDonalds from 9 January, 2013 until his death ~11 months later: he was said to have been a good employee.

Brandon Scott Hole - FedEx Employee

Dean Mellberg (1994 Fairchild Air Force Base shooting) formerly worked as an Air Force member.

Postal killings:
James Hannivan (1926 Adelaide shooting) attacked Adelaide General Post Office, which he was previously fired from.
Rosaire Bilodeau (1934 Quebec shooting / familicide) previously worked at the Quebec postal service.
Joseph M. Harris (1991 Wayne shooting) was an ex-postal worker. Patrick Sherrill (1986 Edmond post office shooting) was a part-time letter carrier.
Curtis Collins (1980 New Orleans shooting) and Floyd Davidson (1975 Gadsden) was a postal employee.
[Way more here.]

Non-shooting mass attacks (vehicle rammings and stabbings):
Abraham Ukbagabir (2015 Västerås IKEA stabbing) was a fisherman.
Wang Hongbin (2011 Gongyi axe attack) was a farmer.
Wu Huanming (2010 Hanzhong kindergarten attack) was the landlord of the school and presumably rented it out.
Chen Kangbing (2010 Hongfu Primary School stabbing) worked as a teacher at a different primary school in Leizhou, but was on sick leave due to mental illness.
Zheng Minsheng (2010 Nanping school massacre) worked as a community doctor.
Yasuaki Uwabe (1999 Shimonoseki Station massacre) worked at two architectural firms before founding his own. He later quit because of "social phobias."
Satoshi Uematsu (2016 Sagamihara stabbings) worked at Tsukui Lily Garden, where the attack took place.
Mamoru Takuma (2001 Osaka school massacre) worked as a hanitor at Ikeda Elementary School, which he attacked, as well as a taxi driver.

Trey Sesler:
Delivering pizzas
Working in grocery stores
Gas station attendant
Working at Yogi Bear’s Jellystone Park
Operating go-Karts and hooking people up to rock walls
Doing odd jobs around his Aunt Liz’s house
Serving ice cream
Reviewing anime on Youtube

Jin Ruchao (2001 Shijiazhuang bombings) worked in a textile factory.

Not all mass shootings, but all were school-related attacks, blatantly ripped off of Wikipedia.
Frank Shugart (1879) was a telegraph operator.
Reuben Pitts (1903, Inman High School) was a teacher.
J. E. Woodward (1904) was a high school principal.
Andrew Kehoe (1927, Bath School) was a school board treasurer, clerk, and electrician.
Verlin Spencer (1940, South Pasadena Junior High School) was a principal.
Matthew Gillespie (1940, Dwight School for Girls) was a janitor.
Tsui Yin (1962, Li Shing junior high school) was a former physical education teacher.
Ralph Best (1963, Paterson School) was a gun engraver.
Robert Mone (1967, St John's RC High School) was a soldier.
Roosevelt Holloman (1977, West Orange High School) was a principal.
Karel Charva (1983 Eppstein school shooting) was a security guard, taxi driver, and alleged psychologist.
Steve Leith (1993, Chelsea High School) was a science teacher.
Charles Raboroko (1999, Anchor Comprehensive High School) was a teacher.
Mohammad Ahman al-Nazari (1997) was a soldier and a bus driver.
Na Teck Ker (1999, Sekolah Menengah Kebangsaan Yoke Kuan) was a fisherman.
Liang Yongcheng (1999) was a school guard.

I'll update this later.

Edit:
Robério Souza de Oliveira (2001, Escola Estadual Aloísio Short) was a farmer.
Yang Zhengming (2002, Number 34 Middle School) was a mini-bus driver.
Chen Peiquan (2003, Yang Gan Middle School) was an English teacher.
John Allen Muhammad (2002 D.C. sniper attacks) was a combat engineer.
Shane Schumerth (2012, Episcopal School of Jacksonville) was a Spanish teacher.
An unknown gunman (2013, Alexander W. Dreyfoos School) was a custodian.

Here's a bunch more sourced from Wikipedia (primary school attackers this time):
Florence, Alabama gunman (1856) was a school master.
George W. Longfelt (Pyfer's School House, 1864) was a teacher.
George T. Cheshire (1867) was a teacher.
Charles J. Gregory (1881) was a teacher.
Droyssig gunman (1902) was a schoolmaster.
Samuel Égly (1904) was a school director.
Heinz Schmidt (1913 Bremen school shooting) was an unemployed teacher.
John Higgins (1923) was a firewood dealer.
Leonard O. Redden (William Reed Elementary School, 1960) was a principal.
Lester Betts (1960) was a mail-carrier and former president of the PTA.
James Underwood (1971) was a school custodian.
Thomas Hamilton (Dunblane Primary School, 1996) was a shopkeeper and scout leader.
Willi Walter Seifert (1964 Cologne school massacre) was a metal worker, sergeant, and worker in a car factory,
An unnamed knifeman (Henan, 1998) was a teacher and soldier.
An unnamed knifewoman (Hajdúhadház, 1998) was a teacher.
Lin Peiqing (Hejiang County, 1998) was a soldier.
Jia Qingyou (No. 1 Experimental Primary School, 2004) was a bus driver.
Liu Hongwen (Chenzhou, 2004) was a teacher.
Yu Hagino (Uji, 2005) was a teacher at a cram school.
Yang Xinlong (Luoying Primary School, 2006) was a farmer.
Charles Carl Roberts (West Nickel Mines School, 2006) was a milk truck driver.
William Michael Stankewicz (North Hopewell-Winterstown Elementary School, 2001) was a former teacher.
Mamoru Takuma (Ikeda Elementary School, 2001) worked at a gas station, in the Japan Air Self Defense Force, school janitor, and taxi drier.
Zheng Minsheng (Nanping City Experimental Elementary School, 2010) was a community doctor.
Jimmy Lee Dykes (Dale County, 2013) was a Vietnam War-era soldier.
Cedric Anderson (2017 North Park Elementary School shooting) was in the US navy and was a hull technician.

Here's a few shooters from Asia:
Yasuhisa Sugiura (2010 Habikino shooting) was a government employee.
Chen Xuerong (1993 Chongqing shooting) was a worker at a machine factory.
Jieming Fan (2013 Shanghai shooting) was a worker at a fine chemical factory.
Wang (1980 Xiwen Handan shooting) was a policeman, brigade militia company commander, and vice chairman of the brigade revolutionary committee.
Zhu Jun (2010 Yongzhou courthouse shooting) was the head of security at the district post office
Woo Bum-kon (1982 Uiryeong spree shooting) was a policeman and worked in the South Korean marines.
Lim Do-bin (2014 South Korea shooting spree) was a soldier in the South Korean military.
Cho Joon-hee (1984) was a private first class in South Korea's 22nd Infantry Division.
Jakrapanth Thomma (2020 Nakhon Ratchasima shootings) was a Sergeant Major 1st Class.
Kim Minchan (2011 Ganghwa Island shooting) was a marine corporal.
Tian Mingjian (1994 Jian'guo Gate mass shooting) was a sharpshooter, regimental staff officer, and first lieutenant.

Here's a few shooters from Russia:
Alexander Lyovin (2007 Balashikha shooting) was a career criminal, contract killer, and bandit.
Oleg Naumov (1998 Sakhalin Oblast shooting) was a private in the Russian Ground Forces.
Sergey Aleksandrovich Pomazun (2013 Belgorod shooting) was an electrician at an electronic warfare battalion.
Ramil Salengalovich Shamsutdinov (2019 Gorny shooting) was a soldier.
Sergey Semidovskiy (2002 Yaroslavsky shooting) was a police captain.
Andrey Shpagonov (1992 Tatarstan shooting) was in the military, worked in a factory, and ran a failed trading business.
Dmitry Vinogradov (2012 Moscow shooting) was a lawyer for the pharmaceutical company that owned the warehouse he attacked.

Oleg Naumov (Sakhalin Oblast, 1998) was a private in the Russian Ground Forces.
Alexander Kuzminykh (Russian submarine Vepr K-157, 1998) was a seaman.
Yaroslavsky shooting (Yaroslavsky, 2002) was a police captain.
Ahmed Ibragimov (1999 Mikenskaya shootings) worked as a driver, ran a small shop, and was a local postman.
Mohamed Lahouaiej-Bouhlel (2016 Nice truck attack) was a driver, ironically.
An unnamed gunman (Borzoy, 2009) was a soldier who attacked other soldiers.
An unnamed gunman (Primorsky, 2008) was a soldier who attacked other soldiers.
An unnamed gunman (Kyakhta, 1992) was a soldier who attacked other soldiers.
An unnamed gunman (Kuressaare, 1972) was a soldier.
4 unnamed gunmen (Chita Oblast, 2005) were soldiers.
Multiple unnamed gunmen (1976 Letipea massacre) were border guards.

Sorry if I'm repeating myself with any of these:
Bjørn Braskerud (Solør, 1938) was a farmer.
Duong Van Mon (Krông Pắk District, 1998) was a rice farmer.
Two unnamed gunmen (Thừa Thiên–Huế Province, 1997) were border guards.
An unnamed gunman (Balagansk, 1997) was a shepherd.
An unnamed gunman (Batyrsky, 1906) was a prison guard.
Vyacheslav Bugakov (Popov Island, 1995) was a border guard.
Oleg Dolzhenko (Zenzevatka, 2000) was a soldier.
Sergey Egorov (Redkino, 2017) was an electrician.
Peter Grachev (Ivankovo, 1925) was a farmer / peasant.
Sergey Semidovskiy (Khorol, 2002) was a police captain.

Mateo Bank was a farmer and also represents the Studebaker car brand in the province of Buenos Aires.

Michael Robert Ryan: gardener-handyman, worked for Newbury Council on a Government Environmental Scheme.

dyonqqr wrote:

Michael Robert Ryan: gardener-handyman, worked for Newbury Council on a Government Environmental Scheme.

He also worked as a caretaker at an All-Girls school

Not a mass shotter, but Timothy Mcveigh worked in the army, and before that he  had a job as a babysitter.

MAB1996 wrote:

Anders Breivik had a farm.

E-I-E-I-O.

The following were also farmers:
Unnamed man (Brazil, 1935)
Unnamed man (Calabar, 1974)
Unnamed man (New Delhi, 1967)
Arvi Lehtikoski
Florentino Basobas
Ben Jebir
Gregorio Caceres
Domingo de la Pena
Severino Mariano de Sena
Kallie Delport
José Emídio
Roque Florindo de Almeida
Arsenio Formentera
Kalle Heikkilä
Idrus bin Hassan
Andrew Kehoe
Petrus Lafras Lombard
Mat Taram bin Sa'al
Joe Pullen
Andres Sacopla
Severino Gomes de Araújo

Alvin Lee King III was an American former high school teacher and farmer.
Peter Grachev was a peasant.
Artur Vaganov was a Russian army sergeant.
Michel Stefanovich Prihatchenko was a former lieutenant and played in a native Russian orchestra at a restaurant.
Sergey Semidovskiy was a police captain.
Almaz Shageyev and Mikhail Sukhorukov were soldiers.
Michael Tselousov "worked for one of the most important posts at school."
Jose Ambata was a police constable.
Mark Stephen Foster was a 4th grde teacher.
Xu Tingjian was a worker at a shoe factory.
Vladislav Chelakh was a soldier.
Anatoly Fedorenko was a Russian army cadet.
William Lepine worked in an orchard, had a gardening and maintenance job for the municipality of Creston, and as a gardener and children’s supervisor at the Dr. Endicott Home for the Retarded.
Yuri Ilyasov was a collective farmer, handyman, and tractor driver.

Magofuku Itsuki (Furuichi, 1796) was a physician.
Tang Yongming (2008 Beijing Drum Tower stabbings): metal presser at the Hangzhou Meter Factory for over twenty years
Jeong Sang-jin (2008 Nonhyeon-dong massacre): Part-time jobs as food delivery man or parking valet
Tomohiro Katō: Auto mechanic, temporary worker at an auto parts factory.

Guilherme Taucci worked in a hot dog stand temporarily, with the mony from this he buy materials for the shotting.