Her mother was Agnes Stedman and her father, Lewis Mercier, was a stockbroker but his enterprise failed.

Mercier's education was improved when a distant relative funded her study in 1904 at Somerville College in Oxford.

It was this fact that made the college's board to require that the principal should have a woman as vice-principal.

[2] Mercier continued to teach in Leeds and Manchester for two years before she became the Principal of Whitelands College[2] in 1918.

In 1930, Mercier and the students moved to new premises designed by Sir Giles Gilbert Scott in Southfields, near Putney.