St. Agnes Academy was founded in 1905 by Mother Pauline Gannon, O.P., Prioress of the Dominican Sisters of Houston, Texas. The Academy opened on February 12, 1906, at its former location at 3901 Fannin. It was in operation at that site until September 1963 when the present-day campus opened at 9000 Bellaire Boulevard in Sharpstown. St. Agnes numbers more than 10,000 graduates, many of whom now have daughters, granddaughters, and even great-granddaughters who are Academy students or alumnae.
In 1917 St. Agnes was accredited by the University of Texas and the State Board of Education. It is now affiliated with the following educational associations: Catholic Schools of the Galveston-Houston Diocese, The National Catholic Educational Association, The Texas Catholic Conference Education Department in conjunction with the Texas Education Agency, AdvancEd (formerly the Southern Association of Colleges and Schools), and The College Board.
From its founding until 1939, the Academy offered 12 years of instruction and boarding facilities for its students. In 1939, the boarding facilities were discontinued, and, in 1954, the Academy began to function as a day school for students in grades 9-12. Today, St. Agnes has approximately 900 girls enrolled.