In 1942 he graduated from San Francisco State College with a degree in speech and drama, and took a job as an announcer with a local radio station. During the Second World Warhe worked as a broadcast correspondent for the U.S. Office of War Information. In 1946 he moved to Los Angeles and resumed his radio career.
He is most remembered as the announcer and good-natured sidekick for Groucho Marx's comedy/quiz show vehicle You Bet Your Life, which began in 1947 on radio and moved to television in 1950, where it remained on NBC for 11 years. Fenneman's mellifluous voice, clean-cut good looks, and gentlemanly manner provided the ideal foil for Marx's zany antics and bawdy ad libs.
"Groucho called [Fenneman] the male Margaret Dumont", according to Frank Ferrante, who portrayed Marx onstage in Groucho: A Life in Revue. "George took it as the highest praise. Groucho called him the perfect straight man." He was also selected because of his intelligence and ability to calculate the scores of the contestants, whom Groucho frequently encouraged to bet odd amounts, making the arithmetic difficult to keep straight on the fly during a live show. Fenneman remained friends with Marx until the latter's death in 1977.
Fenneman was one of a pair of announcers on Dragnet; he shared narration duties with Hal Gibney on radio and the original television series, and then with John Stephenson when Dragnet returned to TV in 1967. It was Fenneman's voice which announced, "The story you are about to see is true. The names have been changed to protect the innocent." while Stephenson would be heard at the end of the episode describing the court trials and verdicts. He was also the principal commercial announcer for the radio version of Gunsmoke, and frequently introduced "Matt Dillon" (William Conrad) after the episode to extoll the virtues of L&M or Chesterfield cigarettes.
He appeared on screen in the 1951 film The Thing from Another World in a substantial but uncredited role. He and his wife were neighbors of the director, Christian Nyby. A spontaneous on-set script revision convinced Fenneman his future was not in movie acting. Producer Howard Hawks took a long scientific speech away from Robert O. Cornthwaite's character Dr. Carrington, preferring to give exposition to a minor character (Fenneman). As a radio performer accustomed to reading from a script and not used to quick memorization, Fenneman stumbled over the technical gobbledegook ("We have the time of arrival on the seisomograph..."), resulting in 27 takes of the scene. In the final film, viewers can see the other actors trying not to smile as Fenneman spouts the lines. He also appeared in an obscure film, Mystery Lake. He avoided on-screen performances thereafter, except as himself, mostly in documentaries.
Fenneman also hosted two game shows: Anybody Can Play in 1958, and Your Surprise Package in 1961. In 1966 he hosted two pilots for Crossword, a game show that would be renamed The Cross-Wits in 1975 and aired with Jack Clark as host. He was the commercial spokesman for Lipton Tea during much of the 1960s, and in that role appeared live on The Ed Sullivan Show when the Beatles made their second U.S. TV appearance on February 16, 1964. In 1963 he hosted an ABC Television program called Your Funny, Funny Films, a precursor to America's Funniest Home Videos. He hosted Talk About Pictures on PBS from 1978-1982. His last credit was as narrator of The Naked Monster, released posthumously in 2005.
Fenneman married Peggy Ann Clifford in 1943 and had three children. He died from emphysema in Los Angeles, California on May 29, 1997 at the age of 77. Oft-repeated statements that Fenneman is the voice of the US Naval Observatory Master Clock or the National Institute of Standards and Technology's radio station WWV are unsubstantiated, according to at least one source.