AMS was founded on September 15, 1947, with eight rooms for Grades 1-6 and 235 students. The school began its first fall term under the guidance of six Sisters of St. Joseph of Carondelet: Sister St. Anne Bromhan, C.S.J., Sister Mary David, C.S.J., Sister Marian Rigali, C.S.J., Sister Eva Francis, C.S.J., Sister Georgine Marie, C.S.J., and Mother Agnes Joseph, C.S.J., the principal.
By 1950, the enrollment grew to 480 students taught by eight sisters and two lay teachers. A four-room addition was built to the north of the original building. Then another four rooms followed in 1954 as the enrollment reached 850. In 1964, four more rooms were added to the middle building. Classrooms now totaled 20 for 1000 students and American Martyrs was the largest elementary school in the Archdiocese of Los Angeles. Over the years, the school decreased in size to eight teaching classrooms with under 300 total students. As population declined, the extra classrooms were transformed into a library, computer laboratory, music room, science laboratory, and learning center, or were used for other parish and school activities.
In the 1990s, the enrollment once again increased and AMS annually added a class for each grade up through seventh grade. In the fall of 2003, the doubling of the classes was completed and two classes of 8th-grade students graduated in June of 2004. In 1994, a faculty room, math lab, and music center were added. In 1998, another renovation added six classrooms, a library, and an extended care center. A parish gymnasium was also completed in 2000. In 2001, the school renovated existing buildings to accommodate the administrative offices, a faculty workroom, and health room. In 2005, a developmental kindergarten was opened to transition students into our school community. In 2010, additional first and second-grade classrooms were added to meet the needs of our growing population, followed by additional third and fourth grades in 2011. A new science and technology wing was opened and an additional 5th grade was added in 2012.