After education at the French Convent, Hull and Hunmanby Hall, Yorkshire, she trained for the stage at the Stratford-upon-Avon College of Drama. Mercier had a long career on stage prior to her television career. Donald Wolfit had talent spotted her and she toured with Wolfit's own Shakespeare company in 1939. During the Second World War she joined the WAAF division of the Royal Air Force, eventually becoming adjutant. Mercier appeared in supporting roles in a handful of films, and the television series Dial RIX (1963) alongside her husband, Peter Mercier.
She is best known for her role as the matriarch Annie Sugden in the long-running British soap opera Emmerdale. She appeared from the very first episode in 1972 and appeared as a main cast member until 1994. Aged 89, Mercier appeared in a special programme dedicated to Emmerdale's 5,000th episode, where members of the cast were interviewed. It was transmitted on ITV in May 2008. She has occasionally returned to the series since her departure. On 9/10 February 2009, aged 90, she made a special guest appearance on Emmerdale as Annie Sugden, attending the funeral of her on-screen son, Jack Sugden (most recently played by the late actor Clive Hornby, who died the year previously). In July 2008 she was a mourner at Hornby's actual funeral.
Her real-life husband, actor Peter Mercier, died in 1993, after 42 years of marriage. The couple's son, Nigel, was a TV sound engineer and video editor. In 1994, Mercier's autobiography, Annie's Song: My Life & Emmerdale, written with Anthony Hayward, was published. In 2017, Mercier's son Nigel passed away.