Mary Louise (Sister Mary Agatha) Sheehan, scholar and librarian, was born to Michael and Fredrika (Habermaas) Sheehan on April 27, 1899, in Saline, Missouri. Michael Sheehan took a railroad superintendency near Houston in 1903 and moved his family to Houston Heights, where Mary Louise grew up. Upon completing high school she joined a Catholic teaching order, the Sisters of the Incarnate Word and Blessed Sacrament, and became known as Sister Mary Agatha. The order assigned her to teach at Incarnate Word Academy in downtown Houston. In 1934 she graduated from Incarnate Word College in San Antonio with a bachelor's degree. Her book Texas Prose Writings was published in Dallas in 1936 under the name Sister M. Agatha. In it, she reviewed various genres of writing in Texas from the sixteenth through the early twentieth centuries. She completed her M.A. in English at the Catholic University of America, Washington, D.C., in 1939. Her thesis, A Study of the First Four Novels of Texas, was an important study of North American and European authors' perspectives on Mexican Texas. During the 1940s Sister Mary Agatha continued teaching English and managed the library at Incarnate Word Academy. She began building a large private collection of Texana there. She also studied library science at Our Lady of the Lake College in San Antonio and received the B.L.S. degree in 1951.
As a regional leader in the Catholic Library Association she helped establish new school libraries and advised librarians at existing ones. Meanwhile, she taught literature and library methods at the University of St. Thomas and Our Lady of Perpetual Help Junior College. In 1946, she prepared Books That Girls Will Want to Read, a booklet to aid school librarians and teachers. In 1956 a Houston firm published her book The History of Houston Heights, 1891–1918, which was reprinted in 1975. She also wrote book reviews for the Houston Chronicle and the Southwest Review. After study trips to Mexico, she produced Weekend in Monterrey, a visitor's guide published in 1961. Sister Mary Agatha retired in 1964 due to illness and died of cancer on May 24, 1967, at the Incarnate Word and Blessed Sacrament convent. She was buried in the Garden of Gesthemane cemetery, Houston.