Bishop Amat High School as 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-instructional high school. The school opened in the classrooms of St. Christopher's School (boys) and St. John's School (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.

The campus site has grown over the years with the present 200 and 500 wings added in 1959. The present 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. Beginning with the gymnasium in 1989 several campus facilities were air-conditioned including the two computer labs, the new physics and biology labs and the Brutocao Library.

The founding principal, Msgr. Thomas Kiefer served from 1957 until 1971, Rev. Roland Zimmerman from 1971-73, Msgr. Jeremiah Murphy from 1973-76, Rev. Thomas Meskill from 1976-78, and Rev. Charles Hill from 1978-81. In 1981, Mr. Merritt Hemenway, a member of the class of '64, 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. In 2001 Msgr. Carroll assumed the dual roles of President and Principal in anticipation of the school's first major Capital Campaign to raise money to construct the Student Activity Center. In July, 2002 Dr. Merritt Hemenway returned as Principal to free Monsignor Carroll to be full-time President.

Since its inception, Bishop Amat has excelled in building Catholic leadership, and in the areas of scholarship, athletics and activities. Christian Service is an inherent element of the school philosophy and students are encouraged to serve the school and community. Bishop Amat has consistently met the standards required for accreditation by the Western Association of Schools and Colleges, and by the Western Catholic Education Association. In 1993, the school received a Six-Year Accreditation, the highest term possible, reflecting the overall excellence of the school program. The foundation of Amat's comprehensive curriculum is a solid core of college preparatory courses with numerous Advanced Placement, Honors and Accelerated courses offered. Students have achieved excellent results in the Academic Decathlon and Academic Olympiad, and have been extremely successful in the National Social Studies, Spanish Language, and Advanced Placement exams. More than 95% of the school's graduates attend college, and many are accepted to the most prestigious colleges and universities in the country.

Lancers typically exemplify a spirit of dedication and self-discipline necessary to accomplish outstanding athletic achievements. Over the years, numerous League and Division Championships have been won by both boys' and girls' teams. In 1994-95, all twelve Lancer team sports qualified for post season competition and, in 1995-96, CIF Championships were won in both Football and Baseball. Each year, Scholar-Athletes are recognized for their outstanding efforts and many are offered scholarships by colleges. Bishop Amat students and faculty have worked hard to maintain the traditions of excellence which continue to give this outstanding high school national recognition for excellence.