The parish was established on February 15, 1925 by Archbishop John Cantwell. The parish was to be called Holy Trinity, in the area that was known as Atwater Park at the time. Before the church building was constructed, a rented house on Atwater Ave. served as the church and the rectory. Later on, mass was said in a vacant store on Glendale Blvd. A portable altar was built by men of the parish and was in there. The church building was completed on October 4, 1925, the first mass at the new location was said. The church was formally dedicated in 1936.
Holy Trinity School opened on September 12, 1949. The school was located in the parish hall which had been divided to accomondate for classrooms. It's original location was adjacent to the church building where the current basketball court is located in the parking lot. The school was under the direction of the Sisters of the Immaculate Heart.
In 1965, Cardinal James Francis McIntyre dedicated the new two-story parish school building. The new parish school was built during the pastorate of Fr. Daniel Gallagher who also built the rectory. He was pastor for 13 years until his death in 1964.
In 1984, under the pastorate of Fr. John Daly, the church had it's air conditioning system installed.
Various Improvements happended during the pastorate of Fr. Maurice Harrigan. The front of the church was redone, a grotto was added to the right side of the rectory. The church was then given a makeover, repainted to its current colors inside and outside the church. The old carpeting of the church was replaced with the current tiles. The new church bell system was installed in the bell tower as well. The parking lot had its lighting upgraded. In 2009, the altar was redone to the current altar. The pipe organ was also replaced that year.
Though many of these changes have happened in recent years, there are some parts of the church that have not changed since it's opening. The hanging lamp lights and windows are still the orginal from 1925. Pictured in the 1962 picture above, is the original altar and pews, the statues pictured are still in their same locations to this day.
As of July 2014, Holy Trinity Church has been under the pastorate of Fr. Ricardo Viveros. The Academy located at St. Casimir Parish and Preparatory school was brought back under one roof in the Holy Trinity school building. In 2016, Holy Trinity was brought into the digital era with the installation of security cameras throughout the property, live streams of special events and masses and lastly, a complete overhaul of the website to its current look. In 2017, the school building was renovated and was upgraded with state of the art technology.