The school's motto, Scientia et Veritas, translates from Latin as Knowledge and Truth.

St Mary's pupils are allocated to Houses, which are named after historical gates of Colchester: St Mary's School was opened in the summer of 1908 by sisters May and Lillian Billson at 15 Lexden Road, a house in St Mary's Terrace East, with eight girls, five of whom had moved from Walton's School in Inglis Road.

Subjects were English, History, Geography, French, Arithmetic, Geometry and Latin.

In 1914 they sat their first public examination, the Junior Cambridge Exam.

In 1923, the school bought new premises on Lexden Road, 'Glen Mervyn'.