The college was established in 1935 by the Sisters of Mercy (Parramatta) in a cottage at 6 Coast Avenue, Cronulla. The earliest known enrolment was sixteen girls. OLMC operated as a small boarding school until 1939 when it was closed during
World War II. It was reopened in 1945 as a small day school, and boarding was discontinued in 1951. In 1959, the Sisters of Mercy purchased the college's current property at Burraneer Bay, which was officially opened and blessed in 1960. In 1972, the college acquired the property on the northern side of Dominic Street. In the early 1980s, the Sisters of Mercy moved out of the Convent, allowing the school to extend into this part of the building. In 1989, the College introduced a new uniform, replacing yellow and brown with the current uniform and school colours of blue, red and white. Extensive building work on the northern side of the grounds was completed in 2003 with the opening of a new multi-purpose centre, The Mercy Centre, along with new music and practice rooms and four general classrooms. Major building work in 2014 and 2015 transformed the Marcy Centre into a performing arts centre with a new stage and two recording studios. The Catherine McAuley building was completed in 2016 and officially opened in 2017, with three science laboratories, four design and technology rooms, five flexible general classrooms and an open
learning space. In 2024 the school accepted their first Year 11 cohort. ==Principals==