Mary Mostert (born 1929) is a Mormon writer and activist.

Mary Mostert was born in 1929 in Arkansas to J.F.T. Mostert and Lucy Kurtz Stallings Mostert. She attended University of Missouri. From 1948 to 1963, she wrote for Nation magazine. From 1965 to 1969, she wrote a weekly column for the Rochester, New York, Times-Union. During the 1960s, she served as a national leader of the Mothers' Lobby. She married H.L. Grooms in 1978. They have six children. For much of her life, she worked to help those disadvantaged people who were not being helped by the government. From 1991 to 1992, she served a Mormon mission to Johannesburg, South Africa. She was the Public Affairs director for the Africa Area for the Church. She is a nationally-respected political writer. She ran unsuccessfully for the New York State Senate and then became a campaign manager. She served as the secretary of Positive Action NOW! a South African women's group that sought to reduce South African social turmoil. She is the author of a few books. She currently lives in Utah.