[2][3] Although biographic accounts relate he began his stage career in Maine, some obituaries (including The New York Times') appear to mistakenly claim he was born in Portland, Oregon.

He appeared in almost one hundred television episodes, including the NBC sitcom, The People's Choice, starring Jackie Cooper, several episodes as police Detective Sawyer, who was being driven nuts by Gracie Allen on The Burns and Allen Show, and three times as a sheriff on the western aviation adventure series, Sky King, before his final appearance, as U.S. President Dwight D. Eisenhower, in Francis Gary Powers: The True Story of the U-2 Spy Incident (1976), a dramatization of the shooting down in 1960 by the former Soviet Union of the U-2 pilot, Francis Gary Powers.

Flavin portrayed Sam Cooper in the 1958 episode, "The Ed Church Case", of the CBS crime drama series, Richard Diamond, Private Detective, starring David Janssen.

From 1960 to 1962, Flavin was cast as Robert Howard in 33 episodes of the ABC/Warner Brothers drama series, The Roaring 20s, starring with Dorothy Provine, Donald May, Rex Reason, John Dehner, Gary Vinson, and Mike Road.

Flavin made his Broadway debut in the 1969 revival of The Front Page, in which he played Murphy and briefly took over the lead role of Walter Burns from Robert Ryan.