Born Tadeusz Wladyslaw Konopka to a Polish American family in Terryville, Connecticut, Knight made one of his earliest screen appearances in an uncredited role as a guard in Alfred Hitchcock's Psycho (1960). Knight's distinctive speaking voice brought him work as an announcer, and he worked frequently in television series such as Bonanza and Get Smart.

His role as the vain and untalented newscaster Ted Baxter in Mary Tyler Moore brought Knight widespread recognition, and his greatest success. He received six Emmy Award nominations for the role, winning the Emmy for "Outstanding Performance by an Actor in a Supporting Role in Comedy", in 1973 and 1976. After the show concluded, Knight appeared in the film Caddyshack (1980), and played the lead role in the series Too Close For Comfort from 1980-1986.

He died from cancer of the urinary tract at the age of 62 and was interred in the Forest Lawn Memorial Park in Glendale, California.