Mary M. Armstrong, cofounder of the Armstrong Browning Library at Baylor University, was born in Buena Vista, Texas, on September 21, 1882, the tenth child of Baptist minister Wilder Richard Maxwell. She received her bachelor of arts degree from Baylor University and did additional work at the University of Chicago. She taught English at East Texas Baptist Institute in Rusk, where her brother was president, and there met A. Joseph Armstrong, a Browning scholar who later became chairman of the English department at Baylor. The couple married in Waco on January 24, 1911, and had one son.
The Armstrongs turned a shared dream into reality by amassing the world's largest collection of Robert Browning and Elizabeth Barrett Browning letters, manuscripts, likenesses, and mementos and raising funds to construct the building that houses them. Together they made twenty-nine trips to Europe to search for Browning materials. During the months of planning and construction of the Armstrong Browning Library, Mrs. Armstrong consulted with architects, builders, designers, and stained-glass experts. Her supervision and expertise are particularly reflected in the library's Elizabeth Barrett Browning Salon. The building was dedicated on December 3, 1951.
Upon her husband's death in 1954, Mary Armstrong assumed the directorship of the library, a post she held until 1959. Thereafter she served as director of the Guardian Angels, a patron group established many years before by her husband. She maintained an office in the library until her retirement in 1966. She was also an active member of the Baylor Round Table and the Domestic Science Club of Waco, as well as a Daughter of the American Revolution and an honorary member of the Waco Woman's Club and the Thursday Club. In 1966 she moved to West Chester, Pennsylvania, to live with her son and his family. She died there on September 1, 1971, and was buried in Oakwood Cemetery, Waco.