Starting his career at ad agency McCann Erickson in 1974, after a year, he became the head writer of the Coca-Cola account in Canada. He then moved from the Toronto office to New York at the request of agency client Coca-Cola.
He was a cast member of the Toronto production of Godspell along with Victor Garber, Martin Short, Eugene Levy, and Andrea Martin. Paul Shaffer of Late Night with David Letterman was the musical director.
He first achieved fame as a cast member of the Canadian syndicated comedy series SCTV, where he portrayed, among other characters, Doug McKenzie of Bob and Doug McKenzie fame. Bob is portrayed by Rick Moranis. Other memorable characters included angry, if succinct editorialist Bill Needle, Scottish scone-chef/bluesman Angus Crock, speedy ad announcer Harvey K-Tel, Lowery organist/curio pitchman Tex Boil and the unforgettable "Cruising Gourmet".
Thomas' first movie was Home to Stay directed by Delbert Mann where Thomas played in a scene with Hollywood legend Henry Fonda. Wrote for and acted in The New Show produced by Lorne Michaels during his hiatus from SNL. Short-lived, this show featured a powerhouse writing staff including Thomas along with Buck Henry, George Meyers, Jack Handey, Tom Gammill and Max Pross, Al Franken and Tom Davis, Jim Downey, Valery Bromfield and Steve Martin. Thomas tried his hand at network television hour-long shows in 1986 when he wrote and co-executive produced Steel Collar man for CBS. The pilot was produced but did not go to series. In 1988 Thomas wrote another hour long show for CBS entitled B Men. This show was produced and the series was back ordered but Thomas took a directing job at Paramount which caused the network to abandon the show. He introduced John Travolta and his wife Kelly Preston when he put them together for the first time in the Paramount film Thomas directed entitled The Experts (1989), while filming in Niagara-on-the-Lake, Ontario.
He wrote for, produced, and starred in the 1990 CBS TV series The Dave Thomas Comedy Show. In 1991 Dave officially starred in the critically acclaimed Showtime comedy, Public Enemy #2 which made Time Magazine's 10 Very Best list for Television that year. In 1992 Thomas tried his hand at reality TV and co-executive produced ABC's America's Funniest People with Vin Di Bona. Thomas left America's Funniest People after thirteen weeks to play a role in Coneheads at the request of buddy Dan Aykroyd.
Starting in 1993 he officially co-starred in ABC's Grace Under Fire with Brett Butler and Tom Poston and continued with the show for 5 seasons. In 1995 Thomas starred in ABC TV movie Picture Perfect with Mary Page Keller and Richard Karn. In 1996 Thomas played the title role in the Fox TV movie Mr. Foster's Field Trip AKA Kiz in the Woods with Julia Duffy. In 1996, Thomas wrote the book SCTV: Behind the Scenes. He appeared in the Red Green Show as Ben Franklin, the American expatriate brother of Dougie Franklin, played by his brother, Ian Thomas. In 1999 Thomas appeared on Cosby with Tom Poston, reprising their father and son roles from Grace Under Fire.
From 1999-2002 Thomas played various roles on the animated series Mission Hill. In 2000 Thomas starred in the theatrical feature Most Valuable Primate. In 2000 Thomas co-starred with Lindsay Felton and Andy Dick in Caitlin's Way. In 2000, Dave Thomas was chosen to host the first annual Canadian Comedy Awards in Toronto.
Thomas co-starred with John Cleese, Rowan Atkinson, Amy Smart, Seth Green, and Jon Lovitz in the Paramount feature Rat Race. As of 2001, Thomas has been the Executive Creative Director of Animax Entertainment , an animation studio based officially in Culver City, CA. In 2001-2002 Thomas appeared with Eugene Levy and Martin Short on Short's show Primetime Glick as Bob Hope. In 2002 Thomas co-starred with Jason Priestly, Dave Foley and Ewen Bremner in Fancy Dancing. In 2003 Thomas played the lead in the Universal feature Beethoven's Fifth also starring John Larroquette, Faith Ford, and Tom Poston.
In 2003 Thomas directed a hospital comedy feature film entitled Whitecoats which he also wrote. The movie starred a group of young Canadian talents including Carly Pope, Peter Oldring, Pat Kelly, Christine Chatelain, and Ingrid Kavelaars. Also rounding out the cast were Thomas himself along with comedy bets Dan Aykroyd, Dave Foley, Matt Frewer, Maury Chaykin, Linda Boyd and Saul Rubinek. As of 2004, Thomas was on the official Advisory Committee for the Comedy program at Humber College, the only such diploma program in the world.
In 2003 Thomas and Moranis reprised the voices of the McKenzie Brothers in Disney's animated feature Brother Bear. Thomas has had a long career doing voices for animation including Animaniacs, Duckman, Catdog, The Adventures of Tarzan, Justice League of America and multiple roles on The Simpsons and Family Guy. In 2004 Thomas co-starred with Marla Sokoloff, Jennifer Tilly, and Barry Watson in the theatrical feature Combo Platter AKA Love on the Side.
In 2006 Thomas reprised his voice role in Brother Bear 2 and appeared as himself in the feature film The Aristocrats. He also started production on ArnoldSpeaks.com a video blog as the voice of Arnold Schwarzenegger.