In 1956 the Christian Brothers founded St. Leo's College as a school for boys. The first Headmaster was Brother Crohan. The Brothers established a strong foundation and the College was highly respected from its early days. The current St. Leo's community is indebted to the Brothers for the extensive facilities, including the two ovals. 

In 1983 the College passed to the control of the Education Office of Sydney. Upon the departure of the Christian Brothers, Mr. Alan Brady was appointed as the first lay Headmaster. At this time, the College became co-educational, with the first girls enrolling in Year 7 and Year 11. Two years later, Mr. Mark Turkington became Headmaster, and led an extensive building program. When the newly created Diocese of Broken Bay was formed, the College came under the jurisdiction of the Catholic Schools Office. 

In 1983 the Brother Berg Building opened, incorporating a multi-purpose hall and extensive art and hospitality facilities. Subsequently, in 1989 the Brother Crohan Library was constructed. 

Upon the appointment of Mr. Terry Blanchard as Headmaster, the College embraced a period of expansion, and significant educational change. The College community flourished, with strong relationships developing between staff, parents and students. Pastoral leadership characterised this period in the life of the College. The vision for a new Technology Centre was developed and construction commenced, with the spectacular new facility opening in 2002.  

Taking up the position as Principal of the College in 2002, Mrs. Debbie Buscall embraced the challenges and opportunities offered by the Whole School Review Process. Initiatives in 2002 included participation in the Quality Teaching Review Process, building upon established College traditions of excellence in Catholic education. Following the Review, a broad consultation process facilitated the development of the College Strategic Plan which outlines the future directions for the College. Action Plans map our progress as a community in meeting the goals set out in the Plan.