Sarton was born in Wondelgem, Belgium (today a part of the city of Ghent), the only child of historian of science George Sarton and his wife, English artist Mabel Eleanor Elwes. When German troops invaded Belgium after the assassination of Archduke Franz Ferdinand in 1914, her family fled to Ipswich, England, where Sarton’s maternal grandmother lived.

One year later, they moved to Boston, Massachusetts, where her father started working at Harvard University. Sarton started theatre lessons in her late teens but continued writing poetry throughout her adolescence. She went to school in Cambridge, Massachusetts, graduating from Cambridge High and Latin School in 1929.

Sarton won a scholarship to Vassar but felt drawn to the theater after seeing Eva Le Gallienne perform in The Cradle Song. She joined Le Gallienne’s Civic Repertory Theatre in New York and spent a year working as an apprentice. However, Sarton continued to write poetry. When she was seventeen, she published a series of sonnets in December 1930, some of which were featured in her first published volume, Encounter in April (1937).

In 1945 in Santa Fe, New Mexico, she met Judith “Judy” Matlack (September 9, 1898–December 22, 1982), who became her partner for the next thirteen years. They separated in 1956, when Sarton’s father died and Sarton moved to Nelson, New Hampshire. Honey in the Hive (1988) is about their relationship. In her memoir At Seventy, Sarton reflected on Judy’s importance in her life and her Unitarian Universalist upbringing. She was elected a fellow of the American Academy of Arts and Sciences in 1958.

Although many of her earlier works, such as Encounter in April, contain vivid erotic female imagery, May Sarton often emphasized in her journals that she didn’t see herself as a “lesbian” writer: “The vision of life in my work is not limited to one segment of humanity…and has little to do with sexual proclivity”. Rather she wanted to touch on what is universally human about love in all its manifestations. When publishing her novel Mrs. Stevens Hears the Mermaids Singing in 1965, she feared that writing openly about lesbianism would lead to a diminution of the previously established value of her work. “The fear of homosexuality is so great that it took courage to write Mrs. Stevens Hears the Mermaids Singing,” she wrote in Journal of a Solitude, “to write a novel about a woman homosexual who is not a sex maniac, a drunkard, a drug-taker, or in any way repulsive, to portray a homosexual who is neither pitiable nor disgusting, without sentimentality …” After the book’s release, many of Sarton’s works began to be studied in university level women’s studies classes, being embraced by feminists and lesbians alike. However, Sarton’s work should not be classified as ‘lesbian literature’ alone, as her works develop many deeply human issues of love, loneliness, aging, nature, self-doubt etc., common to both men and women.

Sarton later moved to York, Maine. In 1990, she was temporarily debilitated by a stroke. Since writing was difficult, she used a tape recorder to record and transcribe her journal Endgame: A Journal of the Seventy-Ninth Year (1992). Despite her physical difficulties, she maintained her sense of independence. Endgame was followed by the journal Encore: A Journal of the Eightieth Year (1993), a celebration of Sarton’s life. She won the Levinson Prize for Poetry in 1993. Her final book, Coming Into Eighty (1995), published after her death, covers the year from July 1993 to August 1994, describing her attitude of gratitude for life as she wrestled with the experience of aging.

She died of breast cancer on July 16, 1995, and is buried in Nelson Cemetery, Nelson, New Hampshire.

Margot Peters’ controversial biography (1998) revealed May Sarton as a complex individual who often struggled in her relationships. A selected edition of Sarton’s letters was edited by Susan Sharman in 1997 and many of Sarton’s papers are held in the New York Public Library.