Ferdin's acting career was primarily in the 1960s, 1970s, and 1980s, though she has appeared in projects sporadically in later years. Ferdin began her career in numerous television series, and gained renown for her work as a voice Actress supplying the voice of Lucy Van Pelt in A Boy Named Charlie Brown (1969), as well as in two other Peanuts television specials.
She had a brief and uncredited role in The Reluctant Astronaut (1965) and was featured in the Walt Disney musical The One and Only, Genuine, Original Family Band (1968). She appeared as Mary Constable in the supernatural thriller Daughter of the Mind and as Abby Clarkson in the horror film The Mephisto Waltz (1971) with Alan Alda. The same year, Ferdin appeared in The Christine Jorgensen Story, based on the life of the first trans woman in the United States to undergo sex reassignment surgery, and in The Beguiled alongside Clint Eastwood and Geraldine Page. She then appeared in the Kurt Vonnegut adaptation Happy Birthday Wanda June, and in the exploitation horror film The Toolbox Murders (1978).
She appeared on the original Star Trek in 1968 as one of a group of orphaned children led by an alien with sinister motives in the episode "And the Children Shall Lead", and in the 1977 series Space Academy as Laura Gentry.
Ferdin provided the voice of Lucy van Pelt in three Peanuts cartoons: the 1969 TV special It Was a Short Summer, Charlie Brown, a 1969 feature film A Boy Named Charlie Brown and the 1971 TV special Play It Again, Charlie Brown. Ferdin was a frequent guest star on episodic television in the 1960s and 1970s, with appearances on Bewitched, Green Acres, The Andy Griffith Show, Branded, Daniel Boone, Custer, The Monkees, The Flying Nun, Gunsmoke, Shazam!, The High Chaparral, Mannix, The Brady Bunch, Family Affair, Love, American Style, Marcus Welby, M.D., Sigmund and the Sea Monsters, Apple's Way, The Streets of San Francisco, Baretta, CHiPS, and 240-Robert.
She later had supporting roles in The Beguiled (1971) with Clint Eastwood and Geraldine Page, and a lead role in the exploitation film The Toolbox Murders (1978) with Cameron Mitchell. She also supplied the voice of Fern Arable in Charlotte's Web (1973). She was to be the voice of Penny in the 1977 Disney film The Rescuers, but was replaced by Michelle Stacy. Ferdin distanced herself from acting in the late 1980s, and shifted her career to animal rights activism, working as an Activist and protester in animal protection programs in New York City and Los Angeles.
Ferdin attended Herbert Hoover High School, graduating in 1977.
Ferdin drew away from acting in the mid-1980s, but did voice the character of Shelley Kelley in the Kids' WB series Detention in 1999.
After leaving her job as a public relations Director in the mid-1990s, Ferdin began working for the Center for Animal Care and Control in New York City. In August 2004, Ferdin accepted the presidency of Stop Huntingdon Animal Cruelty (SHAC), according to statements filed under oath in U.S. District Court in New Jersey. The incumbent, Kevin Kjonaas, resigned after being indicted on charges of conspiracy and interstate stalking. When Kjonaas and six other SHAC Activists were jailed in 2006, Ferdin vowed to continue the campaign. According to salon.com, she defined her role as "a squeaky-clean representative for SHAC USA," but warned that if the SHAC seven were convicted, "[P]eople, I think, are going to get hurt. There's going to be a lot of violence."
In 2004, she accused the parents of Kelly Keen, a three-year-old child killed in a coyote attack, of murdering their daughter and using the story of an animal attack to cover up the crime. This was part of her protest against public efforts to control the coyote population near suburban homes.
In December 2006, Ferdin's group, the Animal Defense League, Los Angeles (ADLLA), announced that it had been awarded $75,000 against the city of Los Angeles for an anti-strategic lawsuit against public participation motion.
In 2008, Ferdin was convicted of contempt of court, after allegedly violating an injunction. The conviction was overturned and she is now suing UCLA for harassment in federal court.
