Mary Ann Hagan was born in Chelsea, Massachusetts into an Irish Catholic family, the fourth of five children. Growing up in Savannah, Georgia, she appeared onstage in numerous high school and community theatre productions. Her first film appearance was in 1976 at age 16. Hagan was watching the filming of İnsan bir defa ölür (1976) in her hometown when the location coordinator approached her and asked if she 'wanted to be in a movie' opposite Burt Reynolds. That led to being cast by director Joan Micklin Silver in a bit part in the Savannah-filmed PBS television movie Bernice Bobs Her Hair (1976) opposite Shelley Duvall.

Years later, Hagan trained at the National Theatre in London in 1990 and then began her professional film/TV career in 1991. She earned her SAG card with the seven words "Holy s**t! What'd ya off him for?" in the sci-fi movie Yaşam Savaşı (1992) ...and then was left on the cutting room floor. She made her series television debut in a 1992 episode of the CBS drama series In the Heat of the Night (1988) and was asked by executive producer Carroll O'Connor to return in two other episodes as different characters. Hagan had supporting roles in several made-for-television movies, including the CBS miniseries A Season in Purgatory (1996), based on Dominick Dunne's eponymous 1993 novel. In it, she appeared as Brian Dennehy and Blair Brown's daughter, a character Dunne based loosely on Ethel Kennedy. Hagan had the lead in Mona's Pets, a short dark comedy that won the Audience Choice Award at the Sante Fe Independent Film Festival. Her favorite roles included being handcuffed to Elizabeth Montgomery in the TV movie With Murder in Mind (1992) and giving birth to twins delivered by Cicely Tyson in the mini-series Oldest Living Confederate Widow Tells All (1994).