St Mary's Anglican Girls' School was founded when two private West Perth girls' schools, Mrs Blanche Gouly's Girls' Grammar School and Miss Emily Hilfirty's The Alexandra High School, amalgamated. St Mary's Church of England Girls' School opened on 14 September 1921. The Rector of St Mary's, The Rev'd C L (Tom) Riley (pictured) was the School's first Principal and Chairman of the Board of Governors. Late in 1921 St Mary's was joined by Miss Shepherd and some of the students from Cowandilla. Other Cowandilla staff and students proceeded to Perth College in Mount Lawley. In September 1923 Roseworthy School, conducted by Miss Edith Tothill, joined St Mary's.

The Early Days

Miss Ethel H Dannatt, Headmistress from 1923-1937, greatly influenced the early development of St Mary's, and today she is remembered in the naming of Dannatt Hall and the Dannatt Bursary.

Following Miss Dannatt's resignation, Miss Katherine C Carter (1938-1940) became Headmistress. Miss Beryl Hamilton was Headmistress 1940-1944. After Miss Hamilton's sudden passing Mrs Elizabeth Myles (1944-1956) took over the reins and steered the School through the post war period. It was during Mrs Theresa Macdonald's (1957-1965) period as Headmistress that the decision was made to move St Mary's from West Perth to Karrinyup.

The Move to Karrinyup

In 1964 a building appeal was launched to develop the present site in Karrinyup. Under the guidance of Mrs Anne Symington (Class of 1933, Headmistress 1965-1982) St Mary's operated in both West Perth and Karrinyup until 1970 when the whole school was finally situated in Karrinyup.

Mrs Audrey Jackson was appointed as Principal in 1983 (1983-1997) and continued the development of facilities with the establishment of the Education Support Unit, the development of the Drama and Music and Art Centres, and the completion of the Bedbrook Redenbach Science Laboratories. The new Junior School was opened in 1993 and the renovations to the Mathematics Teaching Centre were completed for the 1994 school year. The Audrey Jackson Learning Technologies Centre, including the refurbished library and new computer laboratories, was opened during 1996.

In April 1997 Mrs Lynne Thomson became the ninth Principal of the School. In 1998, Anne Symington House underwent refurbishment and renovation to convert St Mary's boarding facilities into first class accommodation. In September 1998, construction commenced on the performing arts centre and on 17 March 2000 The Lady Wardle Performing Arts Centre was officially opened. 

A New Century

In 2003 construction commenced on the Lady Treatt Music and Dance Centre which opened in 2004. The School's eighty-fifth year in 2006 saw significant development of the Junior School with the addition of the Margaret Field Hall, a new pre-primary building, new classrooms and refurbishment of existing buildings to provide a new maths/science laboratory and Learning Support Centre.

The St Mary's Anglican Girls' School Foundation purchased a 100 acre property at Metricup in the heart of the Margaret River Wine Region in 2005 for the School to develop a state-of-the-art outdoor education facility. In May 2007 the building of St Mary's at Metricup: The Lady Treatt Centre for Learning and Leadership began. The Centre opened early in 2008, making St Mary's the first all girls' school in Western Australia to establish such a facility.

In 2011 St Mary's celebrated its 90th birthday, and opened the new Year 7 complex in the Senior School, as well as Hearn Hall. In 2012 the School opened the new Elizabeth Myles Library.