Barth portrayed Myron Fowler, the owner of Peerless Detectives, a rival detective agency in 38 episodes of the CBS television series, Simon & Simon between 1981 and 1989. Additionally, Barth portrayed a police lieutenant on the short-lived 1973 CBS series, Shaft (TV series). He also appeared in regular roles on Night Court, Murder, She Wrote, Civil Wars and Stone.
Barth most well-known voiceover commercial work was for the "Tastes Great...Less Filling" Miller Lite advertising campaign during the 1980s. Barth closed the Miller Lite television commercials by reading the slogan, "Lite Beer from Miller; everything you want from a beer and less". His other voiceover credits included Superman: The Animated Series, Osmosis Jones in 2001 and the 1998 film, Babe: Pig in the City.