A committee of non-conformist merchants and ministers founded the school for boys only in 1807. They located it outside the boundary of London because of "dangers both physical and moral, awaiting youth while passing through the streets of a large, crowded and corrupt city". The school is therefore located in peaceful, secure and rural surroundings, yet by today's standards very close to Central London.
Mill Hill first admitted Sixth Form girls in 1975 and became fully co-educational in 1997.
The school is run by the Mill Hill School Foundation. The foundation offers education to boys and girls aged 3 - 18 in three schools. The foundation's other schools are: