Born on a farm in Albany, New York, during the Great Depression, she began writing in 1949 and sold her first story to Vortex magazine in 1952. 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 the cultural and campaigning lesbian group the Daughters of Bilitis. After her divorce she married numismatist Walter H. Breen on June 3, 1964. They 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. She had known about Breen's sexual interests and previously accepted his sexual affair with a 14 year old boy. Her daughter by Breen, Moira Stern, is a professional harpist and singer. Walter and Marion also had a son, Patrick Breen.

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, including in New York after her move to Staten Island.

Religion

In the 1980s, Bradley was a neopagan but by the 1990s she was a devout Episcopalian 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."

Death

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 scattered at Glastonbury Tor, in Somerset, England, two months later.

Bradley's first published novel-length work was Falcons of Narabedla, first published in the May 1957 issue of Other Worlds. When she was a child, Bradley stated that she enjoyed reading adventure fantasy authors such as Henry Kuttner, Edmond Hamilton, C.L. Moore and Leigh Brackett, especially when they wrote about "the glint of strange suns on worlds that never were and never would be." Her first novel and much of her subsequent work show their influence strongly.

Early in her career, writing as Morgan Ives, Miriam Gardner, John Dexter, and Lee Chapman, Marion Zimmer Bradley produced several works outside the speculative fiction genre, including some gay and lesbian pulp fiction novels. For example, I Am a Lesbian was published in 1962. Though relatively tame by today's standards, they were considered pornographic when published, and for a long time she refused to disclose the titles she wrote under these pseudonyms.

Her 1958 story The Planet Savers introduced the planet of Darkover, which became the setting of a popular series by Bradley and other authors. The Darkover milieu may be considered as either fantasy with science fiction overtones or as science fiction with fantasy overtones, as Darkover is a lost earth colony where psi powers developed to an unusual degree. Bradley wrote many Darkover novels by herself, but in her later years collaborated with other authors for publication; her literary collaborators have continued the series since her death.

Bradley took an active role in science-fiction and fantasy fandom, promoting interaction with professional authors and publishers and making several important contributions to the subculture. In 1966, Bradley became a cofounder of the Society for Creative Anachronism, and is credited with coining the name of that group. In the 1970s, as part of the contemporary wave of enthusiasm for J. R. R. Tolkien's fictional world of Middle-earth, she wrote two short fanfic stories about Arwen and published them in chapbook format; one of them, "The Jewel of Arwen", also appeared in her professional anthology The Best of Marion Zimmer Bradley (1985), although it was dropped from later reprints.

For many years, Bradley actively encouraged Darkover fan fiction and reprinted some of it in commercial Darkover anthologies, continuing to encourage submissions from unpublished authors, but this ended after a dispute with a fan over an unpublished Darkover novel of Bradley's that had similarities to some of the fan's stories. As a result, the novel remained unpublished, and Bradley demanded the cessation of all Darkover fan fiction.

Bradley was also the editor of the long-running Sword and Sorceress anthology series, which encouraged submissions of fantasy stories featuring original and non-traditional heroines from young and upcoming authors. Although she particularly encouraged young female authors, she was not averse to including males in her anthologies. Mercedes Lackey was just one of many authors who first appeared in the anthologies. She also maintained a large family of writers at her home in Berkeley. Ms Bradley was editing the final Sword and Sorceress manuscript up until the week of her death.

Probably her most famous single novel is The Mists of Avalon. A retelling of the Camelot legend from the point of view of Morgaine and Gwenhwyfar, it grew into a series of books; like the Darkover series, the later novels are written with or by other authors and have continued to appear after Bradley's death.

In 2000, she was posthumously awarded the World Fantasy Award for lifetime achievement.

Novels

• Falcons of Narabedla (1957)
• The Door Through Space (1961)
• Seven from the Stars (1961)
• The Colours Of Space (1963)
• Castle Terror (1965)
• Souvenir of Monique (1967)
• Bluebeard's Daughter (1968)
• The Brass Dragon (1970)
• In the Steps of the Master - The Sixth Sense #2 (1973) (based on television series The Sixth Sense, created by Anthony Lawrence)
• The Jewel of Arwen (1974) (novelette)
• The Parting of Arwen (1974) (novelette)
• Can Ellen Be Saved? ([1975]) (adaptation of a teleplay by Emmett Roberts)
• The Endless Voyage (1975)
• Drums of Darkness (1976)
• Ruins of Isis (1978)
• The Catch Trap (1979)
• The Endless Universe (1979) (rewrite of The Endless Voyage)
• The House Between the Worlds (1980)
• Survey Ship (1980)
• The Colors of Space (1983) (unabridged edition)
• Night's Daughter (1985)
• Warrior Woman (1987)
• The Firebrand (1987)
• Black Trillium (1990) (with Julian May and Andre Norton)
• Lady of the Trillium (1995)
• Tiger Burning Bright (1995) (with Mercedes Lackey and Andre Norton)
• The Gratitude of Kings (1997) (with Elisabeth Waters)

Short story collections

• The Dark Intruder and Other Stories (1964)
• The Best of Marion Zimmer Bradley (1985)
• Jamie and Other Stories (1988)
• Marion Zimmer Bradley's Darkover (Darkover collection) (1993)

Series

• Atlantean series
• Web of Light (1983)
• Web of Darkness (1983)
• The Fall of Atlantis (1987) (omnibus edition of Web of Light and Web of Darkness)

• Avalon Series
• The Mists of Avalon (1979)
• Mistress of Magic (audiobook edition of The Mists of Avalon, part 1) (1994)
• The High Queen (audiobook edition of The Mists of Avalon, part 2) (1994)
• The King Stag (audiobook edition of The Mists of Avalon, part 3) (1994)
• The Prisoner in the Oak (audiobook edition of The Mists of Avalon, part 4) (1994)
• The Forest House (1993) (with Diana L. Paxson) (also now known as The Forests of Avalon)
• Lady of Avalon (1997) (with Diana L. Paxson)
• Priestess of Avalon (2000) (with Diana L. Paxson)
• Ancestors of Avalon (2004) (written by Diana L. Paxson)
• Ravens of Avalon (2007) (written by Diana L. Paxson)
• Sword of Avalon (2009) (written by Diana L. Paxson)

• Colin MacLaren series
• Witch Hill (1972)
• The Inheritor (1984)
• Dark Satanic (1988)
• Heartlight (1998)

• Shadow's Gate series (with Rosemary Edghill)
• Ghostlight (1995)
• Witchlight (1996)
• Gravelight (1997)
• Heartlight (1998)

• Darkover series (See: List of Darkover books for complete details)
• The Planet Savers (1958)
• The Sword of Aldones (1962)
• The Bloody Sun (1964)
• Star of Danger (1965)
• The Winds of Darkover (1970)
• The World Wreckers (1971)
• Darkover Landfall (1972)
• The Spell Sword (1974) (with Paul Edwin Zimmer (uncredited))
• The Heritage of Hastur (1975)
• The Shattered Chain (1976)
• The Forbidden Tower (1977)
• Stormqueen! (1978)
• The Bloody Sun (1979) rewritten and expanded edition
• Two To Conquer (1980)
• Sharra's Exile (1981)
• Hawkmistress! (1982)
• Thendara House (1983)
• City of Sorcery (1984)
• The Heirs of Hammerfell (1989)
• Rediscovery (1993) (with Mercedes Lackey)
• Exile's Song (1996) (with Adrienne Martine-Barnes)
• The Shadow Matrix (1997) (with Adrienne Martine-Barnes)
• Traitor's Sun (1999) (with Adrienne Martine-Barnes)
• The Clingfire trilogy
• The Fall of Neskaya (2001) (with Deborah J. Ross)
• Zandru's Forge (2003) (with Deborah J. Ross)
• A Flame in Hali (2004) (with Deborah J. Ross)
• Modern Darkover (also known as The Children of Kings trilogy) (written principally by Deborah J. Ross)
• The Alton Gift (2007)
• Hastur Lord (2010) (written principally by Deborah J. Ross)

• Omnibus editions
• The Children of Hastur (omnibus edition of The Heritage of Hastur and Sharra's Exile) (1982)
• The Oath of Renuciates (omnibus edition of The Shattered Chain and Thendara House) (1984)
• The Darkover Saga (a slipcase set containing Hawkmistress, Sharra's Exile; The Shattered Chain; Stormqueen!; Sword of Chaos) (1984)
• The Ages of Chaos (omnibus edition of Stormqueen! and Hawkmistress!) (2002)
• The Forbidden Circle (omnibus edition of the Spell Sword and The Forbidden Tower ) (2002)
• Heritage And Exile (omnibus edition of The Heritage of Hastur and Sharra's Exile) (2002)
• The Saga of the Renunciates (omnibus edition of The Shattered Chain, Thendara House and City of Sorcery) (2002)
• A World Divided (omnibus edition of Star of Danger, Winds of Darkover and The Bloody Sun) (2003)
• First Contact (omnibus edition of Darkover Landfall and Two to Conquer) (2004)
• To Save a World (omnibus edition of The Planet Savers and World Wreckers) (2004)

• Glenraven series (with Holly Lisle)
• Glenraven (1996)
• In the Rift (1998)

• Survivors series(with Paul Edwin Zimmer)
• Hunters of the Red Moon (1973)
• The Survivors (1979)

Anthologies

• The Best of Marion Zimmer Bradley's Fantasy Magazine (1994)
• The Best of Marion Zimmer Bradley's Fantasy Magazine ... Vol. II (1995) (with Elisabeth Waters)

• Darkover anthologies (edited by Marion Zimmer Bradley, with some short stories by her, but mostly by other writers)
• The Keeper's Price (1980)
• Sword of Chaos (1982)
• Free Amazons of Darkover (1985)
• The Other Side of the Mirror (1987)
• Red Sun of Darkover (1987)
• Four Moons of Darkover (1988)
• Domains of Darkover (1990)
• Renunciates of Darkover (1991)
• Leroni of Darkover (1991)
• Towers of Darkover (1993)
• Snows of Darkover (1994)

• Greyhaven (1983) (with Paul Edwin Zimmer)

• Lythande (1986) (with Vonda N. McIntyre)

• Marion Zimmer Bradley's Fantasy Magazine (1988—2000)
• See Marion Zimmer Bradley's Fantasy Magazine for details.

• Marion Zimmer Bradley's Fantasy Worlds (1998)

• Spells of Wonder (1989)

• Sword and Sorceress series (1984—2008) (edited by Marion Zimmer Bradley, after her death by Elisabeth Waters and Diana L. Paxson)
• See Sword and Sorceress series for details.

Novels under pen names

• Writing under the pseudonym Lee Chapman
• I am a Lesbian (1962)
• Writing under the pseudonym John Dexter
• No Adam for Eve (1966)
• Writing under the pseudonym Miriam Gardner
• My Sister, My Love (1963)
• Twilight Lovers (1964)
• The Strange Women (1967)
• Writing under the pseudonym Morgan Ives
• Spare Her Heaven (1963)
• Anything Goes (1964)
• Knives of Desire (1966)

Poems

• The Maenads (1978)

Music

• Songs from Rivendell (a.k.a. The Rivendell Suite) music and arrangements for several poems from the novel The Lord of the Rings by J.R.R. Tolkien (1960) - reissued as "The Starlit Jewel" CD, available from Flowinglass Music

Editorial positions

• The Darkover Newsletter (1975 to early 1990s)
• Starstone a Darkover fanzine (5 issues 1978-1982)
• Marion Zimmer Bradley's Fantasy Magazine (50 issues 1988 - 2000)

Scholarly work

• Bradley, M.Z. "Feminine equivalents of Greek Love in modern fiction". International Journal of Greek Love, Vol.1, No.1. (1965). Pages 48—58.
• Checklist: A complete, cumulative checklist of lesbian, variant, and homosexual fiction in English (1960).
• A Gay Bibliography (1975).
• The Necessity for Beauty: Robert W. Chambers & the Romantic Tradition (1974)

She also contributed to The Ladder and The Mattachine Review.