Born into a show business family, she is the daughter of Brian Farnon, a musician and conductor and Rita Oehmen, a singer and actress, and sister of Charmian Carr and Darleen Carr. She was raised in Chicago, Illinois.
Farnon, while a teenager, went to theater art classes in Toronto and years later worked on commercial spots. The actress starred in film, television, commercials and cartoons, however her first role was in 1965 in an uncredited role in Burke's Law. She went on to star in supporting roles in multiple other television series, but she was most active in the voice-over field in commercials. While playing 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 famous role, Wonder Woman in Super Friends. She got cast.
From 1973 to 1983, the voice actress portrayed the Amazonian superheroine in 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 the role of Kim Butler in Valley of the Dinosaurs in 1974, but mainly she did the voice of Wonder Woman.
However in 1984, her prominent character was recast in Super Friends: The Legendary Super Powers Show, when the role was given to Constance Caufield and later to B.J. Ward. This case has often been given as an example of injustice in the television and animation industry. Mary McDonald-Lewis voiced the Amazonian superheroine in an episode of Superman.
Shannon Farnon 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.
She was married to William Wells 1966-1975 and had twins, Jeremy and Julie born in 1971. Jeremy died from a skiing accident at the age of 24. This info can be found in her sister Charmian's book, Forever Lisel.