Mt. Alvernia High School began in 1925 as St. James Academy, a small boarding school, and was soon after renamed Mt. Alvernia Academy. It was established by the Allegany Franciscan Sisters to fill the need for the Cambridge University Junior Examinations and the London Royal School of Music Examinations. The number of students attending classes increased steadily over the years.
In 1939 Mother Antoninus, OSF succeeded Mother Theophane as Principal. The school population was only twenty-seven. In 1943 a centre for the Music examinations of the Royal Schools of Music was established at Mt. Alvernia and soon after, the school was considered capable of presenting candidates for the Cambridge School Certificate Examinations.
In 1953, Sr. Eloine Marie became Principal of the school and immediately began working towards having Mt. Alvernia established as a grant-in-aid school. The curriculum was further enlarged and qualified teachers with degrees were employed in order to meet the requirements of the Ministry of Education. Finally in August 1959, Mt. Alvernia became a grant-aided school with an enrollment of 71. Since then, the school population increased even more rapidly and new blocks of classrooms were erected to keep pace with the growing numbers.