Somerville was born and raised in Brooklyn, New York. She started acting and singing at a young age, appearing in high school plays at Poly Prep Country Day School in Bay Ridge, Brooklyn. Somerville attended Boston College as a Musical Theater major, and returned home upon completing her education in order to try acting professionally. At 2 in the morning of October 18, 2008, Somerville and her friend were leaving a party thrown by T-Mobile on Sunset Blvd in Hollywood, CA, when a man approached her and demanded her purse. A struggle ensued, and her friend was shot in the lower back with a .22 caliber handgun. Somerville then gave up her purse, and the shooter fled. He has not yet been apprehended.
Somerville's first acting job was as an extra in the 1996 film City Hall. Her first major role for television was for the lead in the CBS miniseries, Shake, Rattle and Roll: An American Love Story (1999), in which she also sang. Somerville starred in the 2000 series Grosse Pointe, and later played Rachel Hoffman, a colleague of Sandy Cohen, in Season 1 of The O.C..
She had a recurring role on the sitcom Friends as Mona, dating Ross in Season 8. Mona (Bonnie Somerville) first appears to Ross in "The One After 'I Do'" as a co-worker at Monica's restaurant. They later get reacquainted in "The One With Rachel's Date" (airdate October 25, 2001; Season 8, No. 5) and begin dating. Their relationship is troubled from the start due to Ross being the father of Rachel's baby; in "The One With The Stripper", Leonard Green calls her a "tramp" after Ross does not propose to Rachel, and Ross regularly forgets dates with her when Rachel has problems with the baby. They eventually break up in "The One With The Birthing Video" (airdate February 7, 2002; Season 8, No. 15) after Ross does not tell her that Rachel is moving in with him until the baby is born. In "The One With The Tea Leaves" (airdate March 7, 2002; Season 8, No. 17), Ross sneaks into her apartment to recover his "faded salmon color" shirt, which he left there, but has to hide when she arrives home with a date and is discovered when she and her date start making out and he tries to slip out; while Mona is angry with Ross at first, she softens visibly after he apologizes and implies she wouldn't mind getting back together with him, but Ross really only wanted his salmon shirt back, and leaves for good once she hands it over to him.