The history of the school dates back to the beginning of the 18th century, when local Baptist philanthropist Thomas Mills stipulated within his will that money from his estate be used to "further the education of the children in Framlingham",[2] and as such the Trustees of Mills' estate eventually established the first Mills School in 1751 on Brook Lane, Framlingham.

The School was able to survive for one hundred and fifty years, during that time moving from Brook Lane to Double Street within the town.

Opening in 1902 as an independent Grammar school for all children ages 8–16 with a starting total of only twenty-five pupils, Mills Grammar School grew over the course of the 20th century, eventually submitting to local authority control in 1939, changing its age provision to 11–18 in 1945 and finally admitting boys to the school in the mid-Seventies.

By this time the traditional model of Grammar, Modern and Technical schools had fallen out of favour and local councils were encouraged to form all-admitting comprehensive schools for the purpose of secondary education.

Brown became the first headmaster of the newly formed Thomas Mills High School, opened in 1979, on the site of the old Modern School using the existing buildings and facilities, many of which continue to be used today along with many more modern additions.