O’Mara was born Frances Meredith Carroll to John F. Carroll, an RAF flying instructor, and actress Hazel Bainbridge (born 1909/1910 – died 7 January 1998). Her younger sister is actress Belinda Carroll. After boarding school she attended art school before becoming a full-time actress.  O’Mara married Jeremy Young in 1961, but the couple divorced in 1976. She had two sons, Dickon Young (1964–2012) and Chris Linde, but gave up Linde for adoption.

O’Mara made her stage debut in a production of The Merchant of Venice in 1963, although her first film role was some years earlier (under the name Merrie Carroll) in Home and Away (1956) with Jack Warner, as her father, and Kathleen Harrison. Her earliest television appearances, in the 1960s, included guest roles in Danger Man, Adam Adamant Lives!, The Saint, Z-Cars and The Avengers.  In 1970, she appeared in two Hammer Studio horror films: The Vampire Lovers and The Horror of Frankenstein. In the former, she had an erotically charged scene with Ingrid Pitt, in which O’Mara was meant to be seduced; the two women were left laughing on set, however, as Pitt’s fangs kept falling into O’Mara’s cleavage. O’Mara’s work in The Vampire Lovers impressed Hammer enough for them to offer her a contract, which she turned down, fearful of being typecast.

She had a regular role in the BBC drama series The Brothers (1975-76) as Jane Maxwell, and in the early 1980s, O’Mara starred in the BBC soap opera Triangle (1981-82), sometimes counted among the worst television series ever made. She played the villainous Rani in Doctor Who.  The character, as played by O’Mara, appeared in two serials, “The Mark of the Rani” (1985) and “Time and the Rani” (1987) and the “Doctor Who” 30th Anniversary Special “Dimensions in Time” (1993), part of the Children in Need charity event.

Between these appearances in Doctor Who, she played Caress Morrell in the American primetime soap opera Dynasty. As the sister of Alexis Colby (Joan Collins), O’Mara appeared in 17 episodes of the sixth season and 4 episodes of the seventh during 1986. “We had a tremendous bitchy tension between us”, the actress recalled about performing opposite Collins. “My character Caress was like an annoying little mosquito who just kept coming back and biting her.” O’Mara quickly found, however, that she disliked living in California, preferring the change of seasons in Britain, and to her relief was released from her five-year contract after Collins had told the producers that having two brunettes in the series was a bad idea.  After returning to the UK, she was cast as another scheming villain, Laura Wilde, in the BBC soap Howards’ Way (1989-90).

O’Mara continued to make television appearances throughout the 1990s, including Cluedo and Absolutely Fabulous. In 2001, she had a recurring role in the ITV prison drama series Bad Girls before appearing in the short-lived revival of the soap opera Crossroads. She continued to perform on stage and in March 2008 she played Marlene Dietrich in a stage play entitled Lunch with Marlene. From August to November 2008, she played Mrs Cheveley in Oscar Wilde’s stage play An Ideal Husband directed by Peter Hall and produced by Bill Kenwright. She performed in radio and audio plays. In 2000 she reprised her role as the Rani in the BBV audio play The Rani Reaps the Whirlwind, and in 2006 she made a guest appearance in the radio comedy series Nebulous.[10]

One of O’Mara’s sons, Dickon Young (1964–2012), was formerly a stage manager for the Royal Shakespeare Company before setting up his own company building tree-houses in the mid-1990s. He was found hanged, presumed suicide, at the family home in Long Marston, Warwickshire on 31 December 2012.  O’Mara was hospitalised with pneumonia at the time of her son’s death and his body was not discovered for three weeks. She was a vegetarian and animal rights activist.  O’Mara wrote four books, two novels (When She Was Bad (ISBN 0261667157) and Good Time Girl (ISBN 0002242915)), and two autobiographical books, Vamp Until Ready (ISBN 1861057008) and Game Plan: A Woman’s Survival Kit (ISBN 0283060573). In 1993 she married Richard Willis, but the marriage was dissolved in 1996.

Speaking about her bouts of depression, later in her life, O’Mara said: “… I’ve since learnt a cure for depression: listening to J.S. Bach and reading P.G. Wodehouse. This got me through the break-up of my second marriage 17 years ago. The great thing about Wodehouse is that his books are full of romantic problems and yet so hilarious that it puts things in perspective … I’m not frightened of dying, but I love the countryside so much and I’m going to miss it. I’d like to be out in the wind and the trees for ever.”  O’Mara died on 30 March 2014, in a Sussex nursing home, at the age of 74, following a short illness.