St. Mark's School of Texas was formed in 1950 from the merger of two existing schools, Texas Country Day School and Cathedral School for Boys. St. Mark’s traces its origins to Terrill School for Boys, which was founded in 1906 on Swiss Avenue by Menter and Ada Terrill. The school continued there until it moved to the St. Matthew’s Episcopal Church on Ross Avenue in the early 1930s. Terrill foundered during World War II and the school was merged with St. Luke’s School in Austin before being reorganized as the Cathedral School for Boys in the same location in 1946. In 1933 the Texas Country Day School, founded as an alternative to Terrill, opened its first location on the corner of Walnut Hill Lane and Preston Road in Dallas, as a "school in the country for city boys in the tradition of English prep schools." The first graduating class (1935) consisted of one student, Jerry Cunningham. The first headmaster, Kenneth Bouvé, was recruited from Tabor Academy; Bouvé's tenure lasted until 1949. The school moved in 1941 to the present location at 10600 Preston Road but was forced by a fire on November 13, 1943, to relocate to the Fondren Library on the Southern Methodist University campus. Classes returned to the Preston Road site in 1946. Texas Country Day School and the Cathedral School for Boys merged to form St. Mark's School of Texas, founded in 1950.
Both the Texas Education Agency and the Independent Schools Association of the Southwest accredit St. Mark's. The school maintains ties with the Protestant Episcopal Church; indeed, the original charter mandated that there be a chapel and an Episcopal chaplain on the faculty and that the current presiding bishop of Dallas sit on the school's board. The Episcopal Church, however, donates no money for the support of the institution, which remains a non-sectarian school with students from a variety of religious backgrounds. On October 24, 2003, the Texas Historical Commission unveiled a historical marker dedicated to St. Mark’s School of Texas.
In the early 2010s the school library had over 56,000 books and a full media/digital center. The physical plant also included a fine-arts building, tennis courts, a stadium with track, a chapel, a math and science quadrangle, an Olympic-size swimming pool, an older gymnasium, a newer gymnasium donated by Tom Hicks, the Nearburg Hall administration building, a tennis center, and a baseball field. In 2015 the Roosevelt Family Pipe Organ was dedicated in the chapel. New construction during the 2000s included the demolition of the 1940s-era Davis Hall, the original building on campus, making way for the Centennial Hall and the Robert Hoffman ’65 Center. The football field and track complex was renovated with a new artificial surface on the field in the 2000s and again in 2016, as well as a new track surface. In 2019 a new digital scoreboard was installed at the field, and stands were renovated. A new physical science building, the Winn Science Center, was completed in January 2019 with a brand-new planetarium, aviary, and greenhouse, but without the landmark observatory. Following completion of the adjacent Winn Center, the older McDermott-Green Science Building was renovated and rechristened in August 2019.
In 2019 St. Mark's occupied a forty-two-acre campus. The faculty numbered more than 130, of whom about two-thirds hold advanced degrees. The school maintained sixteen "master teacher chairs," six endowed faculty positions, and had an endowment of more than $140 million. Minority enrollment was more than 40 percent, and substantial financial aid was available. As of 2019 the school included 875 boys enrolled in grades one through twelve.