Jill Viola Gascoine was born in Lambeth, London, on April 11 1937. She was educated at Tiffin's Girls School (by her own account, a traumatic experience) and later studied at the Italia Conti Academy of Performing Arts in London. She began her career as a fifteen-year old chorine in pantomime and spent a decade-long apprenticeship singing and dancing in revues and musicals. Married at 28, her first husband was a compulsive gambler who also resented her ambition of becoming an actress and abandoned her and her two children some time during the late 60s. Having to support her family on a single income, Gascoine found work in Glasgow cabaret as a singer and dancer. She was eventually able to get into acting with a repertory company in Dundee and from there (with the aid of theatrical agent Marina Martin) landed parts in popular television dramas including Dr. Finlay's Casebook (1962), Dixon of Dock Green (1955), Softly Softly: Task Force (1969) and Raffles (1975).

Her first recurring TV role was as Letty, the prim, philanthropically-minded wife of taciturn sea captain James Onedin (Peter Gilmore) in The Onedin Line (1971). However, she ultimately became best-known for her role as the emancipated, forthright DI Maggie Forbes in the ITV series The Gentle Touch (1980), the very first British police drama featuring a female (senior) police officer. The concept may well have been inspired by the earlier American series Kadın polis (1974). Publicity claimed that London 'bobbies' wrote to the producers of Gentle Touch, attesting to its authenticity (interesting footnote: the reason why there was never any footage of Maggie actually driving a car was that Gascoine had never learned to drive). There was a later, more action-oriented spin-off, entitled C.A.T.S. Eyes (1985) (akin to a British Charlie'nin melekleri (1976)), with Maggie turned private eye.

In addition to her television work, the actress also performed on the West End stage, including a starring role as Dorothy Brock in a 1987 revival of the musical 42nd Street at The Theatre Royal, Drury Lane. Her younger co-star (as Peggy Sawyer) was Catherine Zeta-Jones. In 1986, Gascoine married fellow London-born actor Alfred Molina and in the 90s made her home in Los Angeles, though she returned to the U.K. on a number of occasions. As her screen work began to wind down, she turned to writing, publishing a trio of novels, respectively, in 1995, 1995 and 1997. Gascoine made an appearance at the Edinburgh Festival Fringe in 2008, whereupon she announced her retirement from acting. Five years later, it was revealed that she had been diagnosed with Alzheimer's Disease. She died at a Los Angeles care facility on 28 April 2020 at the age of 83.