Haworth was born in Hove, Sussex, to a textile magnate father and a mother who trained as a ballet Dancer. She was named Valerie Jill in honour of the day she was born, Victory over Japan Day or V.J. Day. She took ballet lessons at the Sadler's Wells Ballet School to escape from an unhappy home when her parents separated in 1953. Later she attended the Corona Stage School.

Her first film appearance was in the remake of The 39 Steps (1959), directed by Ralph Thomas, when she had a non-speaking part as a schoolgirl. Next she played another schoolgirl in The Brides of Dracula (1960), directed by Terence Fisher.

Haworth only went to the three auditions to get out of school. She was only 15 years old when she appeared as Karen in her first acting role in a feature film. Haworth appeared in the 31 July 1960 issue of Parade magazine. She and Mineo appeared on the front cover of the 12 December 1960 issue of LIFE, part of a photo essay by Gjon Mili.

Mineo and Haworth were also considered for the film David and Lisa (1962), but once again Preminger refused permission. Preminger let her make three French films; Les Mystères de Paris (as Fleur de Marie; 1962), Because, Because of a Woman (fr) (as Cécilia; 1963), and Ton ombre est la mienne (as Sylvie 'Devi' Bergerat; 1963). Haworth co-starred alongside David McCallum in the Outer Limits episode, "The Sixth Finger" (1963). Haworth visited Mineo in Utah in November 1962 and had a nonspeaking role as an extra in The Greatest Story Ever Told (1965).

Under contract to Preminger (for five years), she also worked with him in The Cardinal (1963; as Lalage Menton), and In Harm's Way (1965) as Ensign Anna Lee Dorne, a Nurse Corps officer who, while engaged to Ensign Jeremiah Torrey (Brandon de Wilde), commits suicide after being raped by Captain Paul Eddington, Jr. (Kirk Douglas). Haworth liked working with De Wilde, Patricia Neal, and Douglas, but called John Wayne "the meanest, nastiest man with the worst attitude I ever worked with."

Haworth dated television Producer Aaron Spelling in the summer of 1965, when he was 42 and she was almost 20. Spelling reportedly told friends that he hoped that Haworth would be the next Mrs. Spelling, but Haworth's mother, Nancy, reportedly "scoffed" at the idea.

While filming It! she met Hal Prince, who was doing research for a musical based on Goodbye to Berlin by Christopher Isherwood. Prince asked her if she could sing, to which she claimed, "Louder than Merman." She played Sally Bowles in the original Broadway cast of the 1966 musical Cabaret, a part she played for almost two and a half years. Judi Dench took over the role when the production debuted in London in 1968. Haworth's other stage roles included Bedroom Farce and Butterflies Are Free.

She starred in the horror films It! (1967), The Haunted House of Horror (1969), Tower of Evil (1972), Home for the Holidays (1972), and The Mutations (1974). She only did It! for the money, hated her hair in the film, and hated the film altogether. Haworth liked working with Roddy McDowell, who brought her the poster for the film (on her opening night of Cabaret), and wrote "S-h" in front of the title.

While making Exodus, Haworth became friends, first, and then lovers, with Sal Mineo, and they remained friends until his death in 1976. She lost her virginity to Mineo at the age of 15 in her hotel suite at the Gotham Hotel in Manhattan. Their relationship as a couple ended on Valentine's Day 1964 when she discovered Sal was having an affair with Bobby Sherman. She did not talk to Mineo for a time, but they eventually became friends again, made public appearances together, and in 1971 she invested in his attempted production of The Wrong People (a book about a homosexual relationship with a young boy). Their friendship led to a brief resumption of a sexual relationship in 1969, but after she got pregnant in September and had an abortion, they went back to being just friends. She later told author Michael Michaud that she thought Courtney Burr III, who later had a long-term relationship with Mineo, was the "love of Mineo's life."

Haworth lived on New York City's Upper East Side for many years with her mother. She died of natural causes at the age of 65 on 3 January 2011 in Manhattan. She is buried at Kensico Cemetery.