A Hollywood native, Darby began performing as a child (billed as Derby Zerby) with her parents, who were known professionally as 'The Dancing Zerbies'. She was a teen-ager when she appeared as an extra in the film "Bye Bye Birdie" (1963) and made her speaking debut on an episode of the TV series "Mr. Novak" (NBC, 1964). She had her first speaking role in a film with "Bus Riley's Back in Town" (1965), but did not get her big break until "True Grit". She was rushed into several subsequent films, including "Norwood" (1970), a vehicle to help launch Glen Campbell in films in which Darby played a pregnant and rejected woman he chances to meet. The same year she was the protesting woman whose presence lures Bruce Davison into the anti-war movement in "The Strawberry Statement" (1970). In 1978, Henry Winkler pursued Darby in "The One and Only" and she was the professor dismayed by the changes in Jason Bateman in "Teen Wolf Too" (1987).

While Darby began in TV in the mid-60s on "Mr. Novak" as a high school student with problems, and subsequently appeared in the pilots for both "Ironside" (1967) and "The Streets of San Francisco" (1972), her work on the small screen has been infrequent. She had a supporting role as Virginia Calderwood on the original "Rich Man, Poor Man" miniseries during the 1976-77 season, and also co-starred in "The Last Convertible" (NBC, 1981). Darby made her TV-movie debut with "Don't Be Afraid of the Dark" (ABC, 1973) and was still making occasional TV appearances in the 90s: she had a small role in the children's movie "Secret of the Lizard Woman", a 1995 ABC Saturday Special.