TMI was founded in 1893 by James Steptoe Johnston, the second Bishop of West Texas in the Protestant Episcopal Church.

Johnston was a native Mississippian of the planter class who had fought in twelve engagements in the Civil War, most with the Eleventh Mississippi Regiment.

At the time of the school's foundation, San Antonio lay on the edge of the American frontier, with forts all along the high ground east of the Rio Grande.

The school was then largely funded by donations from wealthy residents of the eastern seaboard, many of whom had heard Johnston speak on the importance of academic and moral education for all young men.

The chapel, designed by alumnus Chris Carson of Ford, Powell & Carson, was recognized as one of the best religious buildings of 2009 by the Interfaith Forum on Religion, Art and Architecture, a knowledge community of the American Institute of Architects.

The TMI Corps of Cadets is under the command of MAJ Joseph Claburn (USA, Ret.

The high school leadership program is affiliated with the United States Army's Junior Reserve Officers' Training Corps.

[11] Due to its high ranking, TMI cadets can apply for places at United States Service academies without the congressional recommendation usually required.