Most Holy Trinity Catholic School's history begins in the spring of 1942 when twenty-three people gathered in a vacant meat market at 7th Street and Dunlap to celebrate the Eucharist. Eventually, the store owner sold the building forcing those faithful Catholics to use a private home for Sunday Mass. When their numbers increased, three acres of land on 7th Street was acquired for the construction of a small chapel called "Mission of Our Lady of the Wayside" which later became the St. Thomas Aquinas Hall which still stands on the north side of McHugh Hall today.

On November 1, 1951 the Bishop of Tucson established the mission as Most Holy Trinity Parish and placed it in the loving hands of the then Reverend Neil P. McHugh to cultivate. In October of 1952, with the future in mind, Father McHugh purchased a citrus grove covering nine-and-a-half acres which would become the future site of  Most Holy Trinity. While his dream of a church, school, and rectory was in the planning stage, Father McHugh put the citrus grove to work selling $1500 worth of grapefruit the first year. The second year, despite the removal of some trees to make way for the school, the grove made a profit of $1000. In 1953 Father McHugh's dream made headlines when they broke ground to build a new, eighty-five thousand dollar school which consisted of four classrooms and an office. This became Most Holy Trinity Catholic School.

That same year, 1953, Sister Dorothy Stang, who then went by the name Sister Mary Joachim, was assigned to the new parish in Sunnyslope, Arizona where she joined Sr. Angelina, Sr. Paula Marie, and Sr. Ann Timothy in opening Most Holy Trinity Catholic School. The elementary school opened its doors on September 1953 with 129 students in grades one through six. Grade seven was added for the 1954-55 school year and grade eight was added for the 1955-56 school year. By 1957 construction on the present church was completed followed by the Rectory in 1962 and finally the convent in 1967,  The Most Holy Trinity Parish was thriving.

Sister Mary Joachim and her fellow Sisters of Notre Dame de Namur began a long tradition of teaching and service at MHT. They reached out to the poor, migrant workers and their families providing necessities and education to those who would otherwise have had none. Those traditions are upheld today in our community through The Society of Our Lady of the Most Holy Trinity. The S.O.L.T. Priests and Sisters were invited to serve the MHT parish and school in 2008 by Bishop Thomas Olmsted. In 2010 under the leadership of Sr. Mary Emmanuel Schmidt, three sisters arrived to assist the school and its mission.  The presence of the S.O.L.T. community is an integral part of the spiritual and academic education our students receive.