St Mary’s now has an international reputation but the school's origins lie in the local community. Its foundation in 1873 was the inspiration of the incumbent vicar of Calne, The Reverend John Duncan, who had a keen interest in education. He felt that the daughters of the middle-ranking Anglican families of the town at that time lacked access to a wide-ranging education. With the support of others in Calne a school was opened on The Green, taking the name from the local church. Nine daughters of the tradespeople and local farming families were the first on the register. Gradually St Mary's expanded under its first superintendents and fellow founders, Miss Ellinor Gabriel and Mrs Penelope Murray.
That St Mary’s sought high academic standards from the start can be seen in early examiners’ reports. A steady increase in numbers, including some younger boys for a period, helped establish its reputation and the enrolment of more boarders from further afield helped secure the school’s finances.
In 1907 St Mary’s moved to its present premises where it was able to acquire more land over the following years. As numbers increased nearby properties were bought as boarding houses and new buildings were established.
Marcia Matthews was Headmistress of St Mary’s over the period covering both world wars. An inspirational leader, she was well-connected to others in the educational world and within the Church. She ensured that her pupils had a thorough education and a good knowledge of events in the wider world. As a result, St Mary’s became well-recognised nationally for the standard of its education and the warm family atmosphere of the school. Each year group has always had its own boarding house while a company system has allowed for different age groups to come together for many joint activities.
By keeping abreast of educational developments over the years St Mary’s has been able to maintain its recognised position. Pupils now come from a wide diversity of backgrounds, and the caring environment of the original foundation lives on.