Heflin, the daughter of a dentist and a homemaker, and sister of the late actor Van Heflin, was born in Oklahoma City and had a 54-year career in theater and TV. She appeared on Broadway with 21-year-old Marlon Brando in 1945 in the original production of I Remember Mama and with Helen Hayes in the London premiere of The Glass Menagerie in 1948. Heflin, whose husband, composer-conductor Sol Kaplan, died in 1990, had three children.
Unlike Mona, Heflin was no quiet hand wringer. "The two things that drove her up a wall were lack of professionalism and prejudice of any kind," says AMC executive producer Felicia Minei Behr. "When she slopped performing five weeks ago, she called me and said, 'Now you're going to have to recast Mona.' She was thinking the show must go on. But I said no—she was Mona, and there will never be another Mona."