Thomas is the eldest son of John and Moreen Thomas and the older brother of Canadian singer-songwriter Ian Thomas. Thomas is married to Kim Thomas and they have four children, two sons, Johnathan and Harrison, and two daughters, Emily and Charlie.
Starting his career as a copywriter at ad agency McCann Erickson in 1974, Thomas became the head writer of the Coca-Cola account in Canada within a year. After watching a Second City stage show in Toronto, and while suffering from self-proclaimed "boredom" in his advertising work, he auditioned for the Second City troupe and was chosen as a performer. 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 five seasons.
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. Also in that year, Thomas and Rick Moranis reprised the voices of the SCTV-appearing 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.