Trinity Catholic was established in 2003 from the merger of St. Thomas Aquinas-Mercy and Rosary high schools to serve north
St. Louis County. Its school building had opened in 1959. The origins of Trinity Catholic began with Mercy High School, established in 1948 as a co-educational institution serving central
St. Louis City and County. Operated by the
Sisters of Mercy, Mercy High School functioned for nearly 40 years until its closure in 1985, when it merged with St. Thomas Aquinas High School in
Florissant. Trinity Catholic High School closed at the conclusion of the 2020–2021 school year. The archdiocese stated that it would be expensive to repair the building, and that area families chose other Catholic schools, which is why the number of students had declined. Former students at the school told Alexander Thompson of the
National Catholic Reporter that they felt betrayed by the school's closure. ==Notable alumni==