O'Mara was born to John F. Carroll, an RAF flying instructor, and actress Hazel Bainbridge (born Edith Marion Bainbridge; 25 January 1910 – 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 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.