Feldon moved to New York after graduating from the famed drama department of Carnegie-Mellon (then Carnegie Institute of Technology) in her native Pittsburgh. She mostly modeled, worked as a chorus girl and appeared in TV commercials until a 1964 episode of "East Side/West Side" brought her to the attention of Hollywood. By the following year, she was cast by Leonard Stern and Mel Brooks in the Cold War/James Bond spoof series "Get Smart", which gave her career enough momentum to remain an active freelancer during the 70s. She appeared in the pilot for "Rowan and Martin's Laugh In" (NBC, 1967), was a regular player on the short-lived "The Marty Feldman Comedy Machine" (ABC, 1972) and, with Jackie Cooper, co-hosted "Dean Martin's Comedy World" (NBC, 1974). Throughout the decade, Feldon also worked as a guest performer on a number of series and found roles in occasional TV-movies. Perhaps one of her better efforts was as Alan Alda's ex-wife dating his best friend Doug McClure in "Playmates" (ABC, 1972). But as it often is for stars of a successful series, Feldon was unable to completely shake the image of Agent 99. In 1989, she and Don Adams teamed for the ABC reunion TV-movie "Get Smart, Again!" and in 1994, they attempted to revive their former glory with the short-lived Fox sitcom update of "Get Smart!"

Feldon's feature career has been sporadic; her best role was the prudish chief of the beauty pageant in Michael Ritchie's underrated "Smile" (1975). Her alto voice and collected screen presence have kept her active as a voice-over specialist in TV commercials and as a host of talk shows, like the syndicated "Special Edition" (1977) and Lifetime's "The 80s Woman". Feldon also performed a one-person show, "Love for Better or Verse", at the York Theatre in New York City in 1995. She also tours the USA offering poetry and prose readings, including one derived from the works and character of Virginia Woolf.