Mary Jane Leach (born 1949) is an American composer based in New York City. She has been a member of the Downtown Ensemble, composer in residence at Sankt Peter, Köln, and has recordings on XI, New World Records, and Lovely Music. In the late 1970s Leach composed mainly with tape, overdubbing her own playing and singing. As her music became more frequently performed she continued writing in an "overdubbing" fashion, layering parts and experimenting with the textures created by multiple voices. Her compositional style is characterized by modality, imitation, and prolongation. Leach received a 1995 Foundation for Contemporary Arts Grants to Artists Award. She currently teaches music courses at Rensselaer Polytechnic Institute.
Leach is also the primary archivist and bibliographer of composer Julius Eastman. She met Eastman in 1981. In the late 1990s, she realized his music had been lost and set about collecting scores and recordings. With Renée Levine Packer, Leach edited Gay Guerrilla: Julius Eastman and His Music, the premiere book on Eastman's life and work.