Quentin was born in Reigate, Surrey,[2] to Kathleen Jones and her husband Fred, a Royal Air Force pilot.

In 1998 she starred in the first sitcom that was specifically built around her: Kiss Me Kate; that year she started the major role of Maggie Mee in the drama Life Begins, which returned for a third series in 2006.

Quentin appeared in the television film Hot Money (2001), which was based on the true story of the theft of hundreds of thousands of pounds from the Bank of England.

ITV has produced five series of the police drama Blue Murder, in which Quentin plays against type in the main role DCI Janine Lewis.

; in a pre-Men Behaving Badly role as a traffic warden in the Mr. Bean episode "The Trouble with Mr. Bean" in 1991; Room 101; Have I Got News for You; and the 2009–10 BBC comedy series Life of Riley, a sitcom about a dysfunctional blended family; and in the BBC Radio 4 improvisational comedy series The Masterson Inheritance, the Radio 4 comedy Any Bloke (starred with Jim Sweeney who was also in The Masterson Inheritance) and the popular BBC Radio 2 sitcom On the Blog.

[13] Quentin starred in the Comedy Central series Big Bad World where she played the role of Jan, the mother of the main character Ben (Blake Harrison).

Starting in 2017, Quentin was co-presenter of the BBC Two programme The World's Most Extraordinary Homes with architect Piers Taylor; 12 episodes were completed and aired.

[18] Her early stage work had also included appearing in the chorus of the original English production of the musical Les Misérables in 1985.

[19] In 2022 she played Mrs Malaprop in the Royal National Theatre's production of Jack Absolute Flies Again, earning her a Laurence Olivier nomination for Best Actress in a Supporting Role.

[20][21] In 2023 she played elder Emma Hamilton and Mrs Cadogan in Jermyn Street Theatre’s world premier production of Infamous by April de Angelis.

[23] Quentin received an Ian Charleson Award commendation for her Masha in The Seagull at the Oxford Theatre Company in 1991.