The name of the school, Cantwell-Sacred Heart of Mary, represents two very distinguished educational traditions long associated with the San Gabriel Valley.

Cantwell High School was established in 1946 by Reverend John J. Cantwell, Archbishop of Los Angeles. With great respect for their teaching ability, the Archbishop enlisted the Christian Brothers of Ireland to staff the school. It was through their hard work that Cantwell produced the quality education and exceptional young men for which the school is known.

Sacred Heart of Mary High School for young women was founded in 1942 by the Religious of the Sacred Heart of Mary, an international apostolic institute of religious women. With a tremendous spirit of faith and zeal, this religious congregation established the school as an expression of their mission to make God known and loved.

In July, 1991 these two institutions came together to form Cantwell-Sacred Heart of Mary High School also known as CSHM.