She was born Margaret M. O'Shaughnessy on June 21, 1931, in Flushing, New York. She received her B.A. from Abertus Magnus College (1953) and her LL.B. from Boston College (1956). Heckler was a member of the Massachusetts Governor's Council from 1963 to 1966, and became involved in electoral politics herself, representing the Tenth District of Massachusetts in the U.S. Congress from 1966 to 1982.

During that time, she founded the Congressional Women's Caucus, which she cochaired from 1978 to 1982. She also served as secretary of health and human services (1983-1985) and as an ambassador to Ireland (1985-1989).