Down was born on 17 March 1954 and raised in Wandsworth, South West London, England. She began acting and modelling, and in her teenage years had won several beauty pageants. She was voted Britain's most beautiful teenager at the age of fifteen. She made her feature film debut in a supporting role of British drama The Smashing Bird I Used to Know in 1969. She later had roles in several other British films, like All the Right Noises, Assault, and Countess Dracula, and well guest starred in television series Six Dates with Barker, Out of the Unknown, and Public Eye.
In 1973, Down was cast as Georgina Worsley, Marchioness of Stockbridge, on the Emmy Award-winning British drama series, Upstairs, Downstairs. This role was her career breakthrough, and after the show ended in 1975 she moved to Hollywood and began her film career. She starred in the 1976 movie The Pink Panther Strikes Again, and later was cast opposite Elizabeth Taylor on the film adaptation of A Little Night Music. Down worked as leading lady in film, and starred opposite Kirk Douglas, Laurence Olivier, Sean Connery, Anthony Hopkins and Donald Sutherland in various films. Her major roles were in The Betsy (1978), The First Great Train Robbery (1979), Hanover Street (1979), Rough Cut (1980), and her box-office bomb Sphinx (1981).
Down has appeared on stage in Hamlet and a musical version of Great Expectations.
Down has played number of leading roles in made for television movies and miniseries. She starred in 1978 British drama The One and Only Phyllis Dixey as Phyllis Dixey. She played the role of Esméralda in a British-American TV movie The Hunchback of Notre Dame in 1982 opposite Anthony Hopkins, and starred in Murder Is Easy (1982), Indiscreet (1988), and Ladykillers (1988). In 1985, she starred in Arch of Triumph with Anthony Hopkins and Donald Pleasence. She starred in the ABC miniseries The Last Days of Pompeii in 1984, and in North and South in 1985. For her role as Madeline Fabray LaMotte in North and South, she was nominated for Golden Globe Award in 1986. She later starred in North and South, Book II (1986), and Heaven & Hell: North & South, Book III (1994). She turned down the roles in Fatal Attraction (later played by Anne Archer) and on The Thorn Birds .
In 1990, Down was cast as series regular for a limited run in the CBS primetime soap opera Dallas as Stephanie Rogers. She earned a quarter of a million dollars' salary for a 10-week shoot.
In the 1990s, Down starred in several small feature and television films, and played guest roles on television series such as The Nanny and Diagnosis: Murder. She starred in the 1994 film Death Wish V: The Face of Death, opposite Charles Bronson, and later appeared with him in the 1995 TV movie Family of Cops. In 1996 Aaron Spelling cast her as Olivia Blake on the NBC soap opera Sunset Beach. The series aired from January 1997 to December 1999. After the soap was cancelled, Down starred in Lifetime movies The Perfect Wife, and You Belong to Me.
In 2003, Down was cast in another soap as Jackie Marone on CBS's The Bold and the Beautiful. In January 2012, Down confirmed that she would be departing the program. Down also appeared in the films The King's Guard with Eric Roberts and Ron Perlman, The Meeksville Ghost, 13th Child, Today You Die, and Seven Days of Grace, on which she also was a writer. In 2011, Down appeared in Victor Salva's thriller film Rosewood Lane with Rose McGowan, Ray Wise, and Lauren Vélez. She later starred alongside Kirsten Vangsness in the comedy film Kill Me, Deadly, and played mother of leads in Dark House, I Am Watching You and Justice.
After ending a ten-year relationship with actor-writer Bruce Robinson, Down married Enrique Gabriel in 1980, but ended their marriage after a year and a half.
Down's second marriage was to film director William Friedkin from 1982 to 1985, with whom she had one son, Jack (born 1982).
She met her third husband, cinematographer Don E. Fauntleroy, during filming of the television miniseries North and South in 1985. They began a relationship, which ended Down's marriage to Friedkin and Fauntleroy's marriage to Susan Ducat. The resulting legal and custody proceedings interrupted the careers of both Down and Fauntleroy for two years and ultimately cost Down and Friedkin US$1 million each. Down and Fauntleroy have a son, George-Edward (b. 1998).
Down has spoken on several occasions about dealing with sexual predators in the film industry. In 2002, she spoke of finding fame in the late 1960s: "The casting couch was in full swing, people expected it... My teen-age years were pretty intense, a lot of pressure and a lot of horrible old men out there". In a 1977 interview, she had also said: "I was promised lots of lovely big film parts by American producers if I went to bed with them. Believe me, the casting couch is no myth".
In 2015, Down discussed her experiences of sexual harassment in the 1970s by an unnamed legendary Hollywood actor and also by producer Sam Spiegel, saying that she "never really enjoyed" her acting career: "Partly that was because of all the lecherous men, studio executives, producers and directors. There was so much running away and hiding under tables. Anyway, I started when I was ten and I’ve been doing it for 50 years."