Farnon, while a teenager went to theater art classes and years later worked on commercial spots. The actress starred in film, TV, commercials and cartoons, however her first role came 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 doing a live-action commercial in 1973 for Flintstone vitamins she was approached by voice director Wally Burr to play what was to be her most famous role, Wonder Woman in Super Friends, she accepted.
From 1973 till 1983, the voice actress portrayed the heroine in Super Friends, Challenge of the Super Friends and The All-New Super Friends Hour. However in 1984, Farnon's prominent character was recast in 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, when producers want to recast a role and give it to someone else and not to the person who has played it for many years.
Shannon Farnon voiced Wonder Woman on many Cartoon Network promos and went on to appear in several films and TV series, but since 2005, her acting appearances have been infrequent. She has married and has children.