Trinity Episcopal School, a parish day school, was founded in 1952 by the Vestry of Trinity Church. It is the third Trinity School to operate on this site. The first school opened for business on July 7, 1845, with 50 students in attendance. It was under the direction of Mr. Dean. The second Trinity began operations in the summer of 1873. Little is known about these early schools except that they represented education in Galveston during the early days when there were no public schools. The present Trinity opened in September, 1952, with 53 students in kindergarten, first, and second grades. In succeeding years, other grades were added and facilities were provided to house them. Moody Hall was built in 1967 to house the newly added fifth and sixth grades. In 1981-82, Trinity launched its Middle School, grades seven and eight. In 1982-83, additional facilities, Trinity II, were opened on the west side of Tremont, directly across from Moody Hall. The Beginning School (two, three, and four year-olds) is housed in Trinity II. Lower school, kindergarten through grade five, is located in Moody Hall and in the Cloister Building.

In the fall of 1990, Trinity added two new buildings to its academic complex. The Lloyd Annex was added to Trinity II for the express use of the PK-2 (two-year-old) class. (In 2013 this building was outfitted for early childhood science education.) Randall Hall was dedicated in December, 1990. The project consisted of classrooms, a science lab, a computer center, a photographic center, and a faculty work area. Trinity III, containing four classrooms and a locker room for the Middle School, was completed in the fall of 1992. During the 2000-2001 school year, additional property was purchased, and permission was gained to buy and close the portion of Winnie Street that had divided the Trinity campus.

An extensive renovation of Randall Hall and the middle school cottages was completed during the summer of 2008, just in time for Hurricane Ike! The storm surge inundated all first floor spaces at the school….and the school made a slow but steady physical recovery throughout that school year. Repairs totaling over $1.3 million were completed in 2009 including the repair of all first floor classrooms. Early in 2012, the School received a $2.7 million grant from the Moody Foundation to build a long-needed gymnasium. The building was completed by May 2013 and named the Robert L. and Ann Moody Activity Center.

In the first half of 2014, the School utilized a subsequent Moody Foundation grant to acquire the half-block at 2215 Church Street. The land was developed as practice field for physical education and athletics and placed into service for the 2015-2016 school year. Today Trinity's campus consists of 23 classrooms, two playgrounds, a lunchroom, a library, an activity center/gymnasium, and a practice field.