The school was founded by the Institute of the Brothers of the Christian Schools, or "Christian Brothers" (sometimes called the "De La Salle Brothers," having been founded by St. Jean-Baptiste de la Salle in France in the 17th century as an organization of Catholic men under vows dedicated to teaching; the term "De La Salle" brothers distinguishes them from the Irish Congregation of Christian Brothers and other, similarly named and purposed groups).
The school was actually founded in Westchester, New York in 1883 as Westchester Institute. Soon thereafter, the school relocated to Clason Point in the Bronx, New York, and was renamed Clason Point Military Academy. In 1926, the school purchased the estate of Frederick Gilbert Bourne, of the Singer sewing machine company, which featured a 110-room mansion, Indian Neck Hall, overlooking Great South Bay, Long Island.
The school had extensive boarding facilities and attracted many sons of the wealthiest Catholic families in the northeastern United States. It also attracted a strong international contingent, particularly from Latin America. For many years LSMA was considered one of the top preparatory schools on the East Coast. It was accredited by the Middle States Association of Colleges and Schools. Its academics were held in the highest regard and although LSMA did not require its students to take Regents exams, they never found this to be a problem. LSMA's reputation was such that its grads were consistently admitted to the best colleges. Graduating classes were often 100% college-bound.
Another academic benefit of not being a Regents program was that the faculty were not bound by a rigid curriculum, which widened the school's latitudes when it came to classes. This benefited the day-to-day education that took place there. For example, in English classes students would often read books that would never have been permitted in a Public School setting (e.g. Kurt Vonnegut, Evelyn Waugh, Bernard Malamud). Honors-track students were often able to take college-level courses (curriculum-wise). LSMA offered some very advanced Math and History classes, as well as Marine Biology.