Forsyth was born in Montréal, Québec, Canada. Her father, David Forsyth, was Scots-Canadian; her mother was an Irish American who worked as a model in New York using her maiden name, Rosemary Collins. Her parents separated when she was an infant, and at 5 years of age she and her mother moved to New York. She studied drama in both high school and college, and she became a model as a teenager. Educated in Stockbridge, Massachusetts, She added to her acting studies by attending the Wynn Handman Drama School in New York. Before she became a model, she worked as a file clerk and a counselor at a camp.
A caption under Forsyth's picture in Life reported, "Rosemary ... was plucked out of a magazine by Universal, then sent to New York for 18 months to act in TV, summer stock, anywhere she could find seasoning jobs."
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.
She starred in The War Lord with Charlton Heston, for which she was nominated for a Golden Globe Award, and the western comedy Texas Across the River opposite Dean Martin.
Forsyth's other notable film credits during the 1960s and '70s include What Ever Happened to Aunt Alice?, Some Kind of a Nut, How Do I Love Thee?, The Brotherhood of the Bell, City Beneath the Sea, Black Eye and opposite Heston again in the disaster film Gray Lady Down.
By 1971, she started showing up in television frequently. She starred in a pilot for a 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,:512 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.
In the early 1960s, Forsyth was the second actress to play Joan Miller on The Defenders.:248 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 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.
Forsyth was married to actor Michael Tolan. In 1972, she married Ron Waranch. In 1980, she married Alan Skip Horwits.
In 1966, Forsyth was nominated for the Golden Globe Award "New Star of the Year-Actress" for her work in Shenandoah.Shenandoah (1965) as Jennie Anderson
The War Lord (1965) as Bronwyn
Texas Across the River (1966) as Phoebe
It Takes a Thief (in episode "When Good Friends Get Together", 1968) as Miss Harris
Where It's At (1969) as Diana Mayhew Smith
What Ever Happened to Aunt Alice? (1969) as Harriet Vaughn
Some Kind of a Nut (1969) as Pamela Anders
How Do I Love Thee? (1970) as Marion Waltz
The Brotherhood of the Bell (1970) as Vivian Masters Patterson
City Beneath the Sea (1971) as Lia Holmes
Is There a Doctor in the House (1971) as Dr. Michael Griffin
The Death of Me Yet (1971) as Sybil Towers
Night Gallery (in segment "Deliveries in the Rear", 1972) (Season 2, Ep 53) as Barbara Bennet
Columbo: Murder by the Book (1971) (directed by Steven Spielberg)
One Little Indian (1973) (cameo)
Black Eye (1974) as Miss Francis
Mannix - Portrait of a Shadow Nov. 1974. Season 8 Episode 9
Kung Fu (1974) as Ellie Crowell in the episode 'A Small Beheading' (Season 3, Episode 42)
My Father's House (1975) as Judith Lindholm
Days of Our Lives (1965) as Laura Spencer Horton (unknown episodes, 1976-1980)
Gray Lady Down (1978) as Vickie
Fantasy Island as Dr. Melanie Elisabeth Griffin (3 episodes, 1980-1983)
WKRP in Cincinnati (1981) as Joyce Armor
Magnum, P.I. (in episode "Legacy from a Friend", 1983) as Margaret Chase
Santa Barbara as Sophia Wayne Capwell (40 episodes, 1984)
Remington Steele as Marjorie Flowers (1 episode, 1984)
Murder, She Wrote as Andrea Reed (in episode "My Johnny Lies Over the Ocean", 1985)
Dallas as Ann McFadden (3 episodes, 1985)
Mr. Belvedere as Ilsa Hufnagel (1 episode) and Louise (2 episodes—2 part finale 1990)
The Gladiator (1986) as Loretta Simpson
A Case for Murder (1993) as Judge Helen McCoy
Exit to Eden (1994) as Mrs. Brady
Disclosure (1994) as Stephanie Kaplan
Melissa (1995) as Dr. Gardner
The Other Women (1995) as Dr. Angela Crane
Daylight (1996) as Ms. London
JAG as Senator Grace Marion (1 episode, 1996)
Star Trek: Voyager as Alzen (1 episode, 1997)
Dharma & Greg as Merrill (2 episodes, 1997-1998)
Girl (1998) as Mom
Valerie Flake (1999) as Irene Flake
Chicago Hope as Dr. Edith Strauss (2 episodes, 1997-2000)
ER as Judy (1 episode, 2000)
Ghosts of Mars (2001) as Inquisitor
Ally McBeal as Judge Martha Graves (2 episodes, 2000-2001)]
Providence as Elaine (1 episode, 2002)
A Time to Remember (2003) as Dorothy Walderson
Monk as Marcia Ellison (1 episode, 2004)
NYPD Blue as Felicia Heilbrenner (1 episode, 2005)
Without a Trace as Martha Scoggins (2 episodes, 2002-2005)
Sweet Nothing in My Ear (2008) as Louise MIller