Rosemary Cox was born on August 2, 1939. She graduated from Swarthmore High School in 1957, and Mount Holyoke College in 1961. She received an L.L.B. from Harvard Law School in 1964. On June 16, 1962 she married Jon Joseph Masters, also a student at Harvard Law School. They had a daughter, Brooke Alison, in 1967, and a son, Blake Edward, in 1970. She worked as the Administrative Diector of the Manhattan Bowery Project from 1967-1968. She also worked as a legal consultant at the Vera Institute of Justice in 1969.