Widdoes graduated from The Loomis Chaffee School in Windsor, Connecticut in 1972, and is a member of their Board of Trustees. He began his acting career during college, starring in a production of The New Amen Show at the Diners Playhouse in Lexington, Kentucky in 1974. He next attended Skidmore College in 1972 and then transferred to New York University's Tisch School of Arts, graduating in 1976 with a Bachelor of Fine Arts degree. While in New York, he roomed with such soon-to-be famous actors as Michael O'Keefe from Caddyshack and The Great Santini. He then began performing on New York City stages in productions such as the 1977 Equity Library Theatre revival of Wonderful Town and the 1982 Broadway musical Is there life after high school? His role in this last production won him a Theatre World Award.

Widdoes starred as senior student and fraternity President Robert Hoover alongside John Belushi in the 1978 film National Lampoon's Animal House as well as the 1979 TV series spin-off Delta House. He has guest starred in many TV series episodes since then, including Remington Steele, Night Court, Dave's World, and My Wife and Kids. Widdoes starred as the patriarch of the Pembroke family, Stan, in the 1984-1985 seasons of Charles In Charge.

Beginning in 1998, he produced and/or directed many episodes for various television series. Some of these included Just the Ten of Us, Empty Nest, Anything But Love, Harry and the Hendersons, Boston Common, Brother's Keeper, Reba, 8 Simple Rules (For Dating My Teenage Daughter), The King of Queens, 'Til Death, Mom and The Bill Engvall Show. He directed several episodes of Two and a Half Men during the show's first six seasons, and began directing the series regularly beginning with season seven. He directed all but two episodes of the series' later six seasons.