The actress starred in film, television, commercials and cartoons, however her first role was in 1965 in an uncredited role on Burke's Law. She went on to appear in multiple other television series, but was most active in the voice-over field in commercials. While playing a mother in a live-action commercial in 1973 for Flintstone vitamins she was approached by voice director Wally Burr to audition for what was to be her most long-running famous role, Wonder Woman on Super Friends.
From 1973 to 1983, Farnon voiced the Amazonian superheroine on Super Friends, Challenge of the Super Friends, The All-New Super Friends Hour, The World's Greatest Super Friends and the revival of Super Friends. Farnon did voice several incidental characters, such as Lois Lane and Hawkgirl on some of the series and also played Kim Butler in Valley of the Dinosaurs in 1974, but mainly she did the voice of Wonder Woman.
She voiced Wonder Woman on many Cartoon Network promos and went on to appear in several films and television series, but since 2005, her acting appearances have been infrequent. She has voiced various commercials, including Betty Crocker, Nivea, Kodak, National World War II Memorial, Oil of Olay, Scotchgard and Cartoon Network spoofs of The Super Friends.