Somerville was born in Iowa City, Iowa to Lefa Mary (née Pash; 1918–2011) and Rev. Paul Somerville (1919-1995), who moved the family around as he preached at different parishes, until settling in Traer, and eventually Cresco, Iowa. There, her mother worked as a librarian and Phyllis attended high school where she was involved with cheerleading, drama, and music, including playing clarinet in the All-State band. She has three brothers: Bruce (also an actor), Paul (1947-2005) and Stephen, a pathologist.
She attended Morningside College and then transferred to the University of Northern Iowa, studying theatre and graduating with a degree in English in 1966. In college she starred in productions such as The Visit, Electra, and Macbeth. She next did graduate work performing classical roles like Puck in A Midsummer Night's Dream at the Hilberry Theatre through Wayne State University in Detroit and later became a resident equity Actress with the Arena Stage in Washington D.C. Her first paid job as an entertainer was at Buckskin Joe amusement park over summer break in college.
Somerville made her Broadway debut as Wilma in the 1974 musical Over Here! which was nominated for the Tony Award for Best Musical. The following year, she performed in the off-Broadway musical The Journey of Snow White as "The Witch". She appeared in the 1978 Broadway production of Once in a Lifetime. In 1983 she was the understudy for Kathy Bates in the Broadway production of Marsha Norman's 'night, Mother, and later toured in the role opposite Mercedes McCambridge. In 1984, she originated the role of Glory in Norman's next play, Traveler in the Dark, at the American Repertory Theater.
In 1990, she originated the role of Joyce in The Sum of Us by David Stevens. In 2001, she originated the role of Hannah in the Off-Broadway musical The Spitfire Grill, a role played in the 1996 film by Ellen Burstyn. In 2009 she performed in the musical Happiness at the Lincoln Center Theatre. She has appeared in other productions Off-Broadway, as well as in regional theatre, including many roles with the Hartford Stage Company. She is a member of the LAByrinth Theater Company. In 2014, she appeared Off-Broadway in an experimental all-female production of I Remember Mama, alongside Rita Gardner, Heather MacRae and Barbara Barrie.
