Bishop Amat Memorial High School was built by the Archdiocese of Los Angeles in 1957 to serve the Catholic families of the San Gabriel Valley. Funded through the Cardinal McIntyre Youth Education Fund, Bishop Amat was founded as a co-educational high school. The school opened in the classrooms of St. Christopher’s parish (boys) and St. John’s parish (girls) in September 1957. On December 16, 1957 the present campus site was opened. His Eminence, James Cardinal McIntyre formally dedicated the campus in October 1959, dedicating the school to the memory of Bishop Thaddeus Amat.
The original faculty and staff were composed primarily of men and women religious. Women from the Sisters of the Immaculate Heart, Sisters of St. Louis, Sisters of the Sacred Heart, Carmelites, Benedictine Sisters, and Franciscan Sisters taught the girls. Fathers of the Sacred Heart of Jesus staffed the boy's department. The early faculty had very few lay teachers.
The founding principal, Msgr. Thomas Kiefer, served from 1957 until 1971; Msgr. Roland Zimmerman from 1971-1973; Msgr. Jeremiah Murphy from 1973-1976; Rev. Thomas Meskill from 1976-1978; and Rev. Charles Hill from 1978-1981.
In 1981, Mr. Merritt Hemenway, a member of the Class of 1964, was appointed as the first lay principal of the school. He was succeeded in 1990 by Rev. James Anguiano who served until 1992 at which time, Msgr. Aidan M. Carroll was appointed. On January 1, 2001, Msgr. Carroll was named as President/Principal of the school. In July 2002, Dr. Merritt Hemenway was named Principal and Msgr. Carroll, President. In 2012, Mr. Richard Beck was named Principal.
The campus site has grown over the years with the present 200 and 500 wings added in 1959. The gymnasium was added in 1962, the stadium in 1965 and the 300 wing in 1966. In 1973, the school formally became co-educational, with a single administrative structure. A building dedicated to the performing arts was completed in 1982. Four classrooms, including a full computer lab, containing the latest technology available, were completed in 1988. A second computer lab was added in 1992, a new state of the art biology lab in 1993, a refurbished physics lab in 1994 and the Brutocao Library in 1995. In 2000, a new covered lunch structure was built complete with an integrated sound system and barbeque pit.
The construction of a Student Activity Center was completed in April 2004. The facility houses a pavilion that seats 1600 for athletic contests and other events, and it includes a weight room/fitness center, a dance studio, lunch facilities for serving student meals, locker rooms including a varsity room for football and baseball, team rooms for volleyball and basketball, a training room for students and facilities for coaches and athletic personnel. In 2008 the facility was named for Monsignor Aidan Carroll who served Amat as Principal for ten years and has been the school's first President. Monsignor Carroll was instrumental in the concept, development and procurement of funds to pay for the facility.