At an early age, this stage and screen actress began her career by making several television appearances. She appeared twice in the series Coronation Street (1974, 1976); enacted the character of Angela Reed in the Emmerdale Farm series in 1977; and played Maureen Maskell in the episode "Shot Gun" in the Juliet Bravo series (1980). She played Dany in The Gentle Touch (1982), the character Ingrid Rotherwell in A Kind of Loving (1982), and Christine Bolton in the "Always Leave Them Laughing" episode of Bergerac (1983).
At age 18, this eye-catching brunette, later to be chosen by People magazine as one of the 50 Most Beautiful People (1991), appeared at the Royal Court Theatre in England, and in her first film role, as one of the groupies chasing after rock star Bob Geldof in the musical Pink Floyd: The Wall (1982).
After two more television appearances, as Ulla in Reilly: The Ace of Spies (1983) and in A Christmas Carol (1984), Whalley had roles in Peter Smith's comedy No Surrender (1985) as Cheryl; Mike Newell's crime story Dance With a Stranger (1985) as Christine; as Mary Hall in Newell's drama The Good Father (1987); and in Will You Love Me Tomorrow? (1987).
Whalley was outstanding as the beautiful Nurse Mills in the TV miniseries The Singing Detective (1986), and was nominated for a BAFTA Award for Best Actress for her role as Emma Craven in the Edge of Darkness TV miniseries (1986).
While enacting the part of Sorsha in the 1988 fantasy film Willow, a romance developed with her co-star Val Kilmer and they were married in March of that year. Their family grew to include a son and daughter. Whalley immediately began to be credited by her new hyphenated last name in the thrillers To Kill A Priest (1988) and Kill Me Again (1989). She gave a star-caliber performance as the infamous Christine Keeler, a central figure in the Profumo affair, in Scandal (1989).
After her elegant appearance as Beatrice in the television piece A TV Dante: The Inferno Cantos I-VIII (1989), there followed two roles in the best-forgotten movies Navy SEALS (1990) and The Big Man (1990). However, Whalley was brilliant as Jenny Scott in the mystery thriller Shattered (1991).
Many roles soon followed in Storyville (1992), The Secret Rapture (1993), Mother's Boys (1993), A Good Man in Africa (1994), Trial by Jury (1994), and she took over Vivien Leigh's classic role of Scarlett O'Hara in the TV sequel to Gone With the Wind entitled Scarlett (1995).
In February 1996, Kilmer and Whalley were divorced and, except for Run the Wild Fields (2000), she went back to using her maiden name. She appeared as Lorelei in the comedy The Man Who Knew Too Little (1997), A Texas Funeral (1999), The Guilty (2000), and convincingly re-created the persona of Jackie Bouvier Kennedy Onassis in the TV miniseries (2000). Numerous film roles followed over the course of the next decade, including appearances in 44 Inch Chest and Francis Ford Coppola's Twixt, and in 2011 Whalley made a conspicuous return to television with recurring roles on both Gossip Girl and The Borgias.