DeMatha Catholic High School first opened its doors on September 9, 1946 with an enrollment of only 18 students.
Under the direction and leadership of the Very Rev. F. Cyprian Nusca, O.SS.T., Provincial, the Very Rev. F. Michael Nardone, O.SS.T., Superior and first principal, and Rev. Paul Donovan, O.SS.T., Registrar and Dean, all priests of the Order of the Most Holy Trinity (Trinitarians), the primary focus of the school was to educate and prepare those interested in becoming clergy in the Trinitarian Order. However, due to a lack of Catholic high schools and facilities in the metropolitan area (and Hyattsville), Catholic parents requested that their sons be granted permission to enroll at DeMatha. Following their tradition of service to the community and to the Archdiocese of Washington, the Trinitarians agreed and began accepting a limited number of lay students.
The student body was suddenly over 80 students at the opening of school in 1947. Quick changes were needed. Since school was taught in the Monastery and that building was no longer sufficient for the larger enrollment, a six-car garage was rapidly remodeled to provide desperately needed classrooms. This building quickly gained the apt nickname "Fort Necessity" and is currently the home of the Fotos Arts Center.
DeMatha's first graduating class in 1948 consisted of four students. Work began in 1949 on a new building specifically designed as a secondary school, and in January 1950 (despite a flu epidemic at the time), the first classes were held in this new facility. The make-up of the new structure, now known as the "old wing", was quite different than it is today, containing a cafeteria (originally to be an indoor pool), chapel, library, laboratory, classrooms, and a large auditorium-gymnasium. The third floor, which was to contain additional classroom space, was not completed until the following year. Changes in curriculum were made so that high academic standards could be maintained and state requirements fulfilled. In 1954, DeMatha Catholic High School was placed on the list of state-approved non-public schools. DeMatha's first decade of existence was complete and a tradition of dedication, service, and excellence had begun.