Mary Cantwell was born in Providence, Rhode Island in 1930. She grew up in Bristol, Rhode Island in a largely Protestant neighborhood. Cantwell was brought up Catholic and was singled out in her largely Protestant school. Cantwell graduated from Colt Memorial High School in 1949.

Cantwell earned her B.A. from Connecticut College in 1953 and married her husband in December of that year. The couple had two daughters. They would later divorce.

Cantwell worked as a copywriter at Mademoiselle Magazine until 1958. Between 1958 and 1959, she worked as a feature writer for Vogue. Cantwell returned to Mademoiselle in 1962 and was promoted to chief copywriter.

Cantwell's work at the New York Times began in 1980 and included editorials and essays for that newspaper. While at the New York Times, Cantwell wrote a regular column entitled Hers. Cantwell was awarded the Connecticut College Medal in 1983. She also received the Walker Stone Award from the Scripps Howard Foundation in 1986.

Cantwell also published three personal memoirs during the 1990s. (See selected bibliography below)