In the early 1960s, Forsyth was the second Actress to play Joan Miller on The Defenders. She portrayed Sophia Wayne Capwell on Santa Barbara (1984). She played the fourth Laura Spencer Horton on Days of Our Lives, from 1976–1980, and appeared as Ann McFadden on Dallas (1985).

She made her acting debut in 1963 on the TV series Route 66 as Claire in episode No. 101, "I Wouldn't Start from Here". She made her film debut in 1965 in Shenandoah from Universal Pictures as James Stewart's daughter.

In 1966, Forsyth was nominated for a Golden Globe Award as New Star of the Year-Actress for her work in Shenandoah.

By 1971, she started appearing on television frequently. She starred in a pilot for the television series Is There a Doctor in the House?, about a young city Doctor who moves to the country to work with a crusty older Doctor played by william Windom, but the series was not picked up. From the mid-1970s and on, she worked primarily in television. She was featured in the Columbo television series episode "Murder by the Book," directed by Steven Spielberg. She was also in several episodes of Mannix.

Forsyth was married to actor Michael Tolan. In 1972, she married Ron Waranch. In 1980, she married Alan Skip Horwits.

She guest-starred on such television shows as Fantasy Island, Magnum, P.I., Remington Steele, JAG, Star Trek: Voyager, Murder, She Wrote, Chicago Hope and ER, as well as appearing in the films The Gladiator (1986), A Case for Murder (1993), Exit to Eden (1994), Daylight (1996) and Girl (1998). She had a small but pivotal role in 1994's Disclosure, a film starring Michael Douglas based on a novel by Michael Crichton.