Born Marion Eleanor Zimmer on June 3, 1930, she lived on a farm in Albany, New York, and began writing at the age of 17. She was married to Robert Alden Bradley from October 26, 1949 until their divorce on May 19, 1964. They had a son, David Robert Bradley (1950–2008). During the 1950s she was introduced to lesbian advocacy organization the Daughters of Bilitis.
After her divorce, Bradley married numismatist Walter H. Breen on June 3, 1964. They had a daughter, Moira Greyland, who is a professional harpist and singer, and a son, Mark Greyland.
In 1965, Bradley graduated with a Bachelor of Arts degree from Hardin-Simmons University in Abilene, Texas. Afterward, she moved to Berkeley, California, to pursue graduate studies at the University of California, Berkeley between 1965 and 1967. In 1966, she helped found and named the Society for Creative Anachronism and was involved in developing several local groups, some in New York after her move to Staten Island.
Bradley and Breen separated in 1979 but remained married, and continued a business relationship and lived on the same street for over a decade. They officially divorced on May 9, 1990, the year Breen was arrested on child molestation charges after a 13-year-old boy reported that Breen had been molesting him for four years. She had edited Breen's book Greek Love, which was dedicated to her, and in 1965 had contributed an article, "Feminine Equivalents of Greek Love in Modern Literature", to Breen's journal The International Journal of Greek Love. She had known about Breen's sexual interests and previously accepted his sexual abuse of a 14-year-old boy.
While she was attending the College for Teachers (now University at Albany, SUNY) in Albany, Bradley became involved in Western esoteric tradition. She later completed the Rosicrucian correspondence course.
In the late 1950s or early 1960s, Bradley and Walter H. Breen founded the Aquarian Order of the Restoration based on the work of Dion Fortune. By 1961 she was formally initiating others, including Ramfis S. Firethorn.:313
Bradley was active in Darkmoon Circle, which was founded in 1978 by several women who were members of her Aquarian Order of the Restoration. Bradley renovated her garage to provide a meeting room for Darkmoon Circle as well as for other local Pagan groups. In 1981 Bradley, Diana L. Paxson, and Elisabeth Waters incorporated the Center for Non-Traditional Religion.
In the 1990s Bradley said she would return to Christianity, telling an interviewer: "I just go regularly to the Episcopalian church... That pagan thing... I feel that I've gotten past it. I would like people to explore the possibilities."
After suffering declining health for years, Bradley died at Alta Bates Medical Center in Berkeley on September 25, 1999, four days after suffering a debilitating heart attack. Her ashes were later scattered at Glastonbury Tor in Somerset, England.:28-29