In 1871, General Sylvanus Thayer, wrote a will which included a bequest to build “an academy in which young persons of the male sex (or both male and female if my trustees deem it expedient) be educated…to promote the cause of education in the Commonwealth and to benefit the town of Braintree, the place of my birth”. The year after, Thayer died and his money was utilized by his trustees to establish the new school.
The first building was completed in 1876, and the academy opened that next year with tuition free for students of Braintree and $75/year for all others. The Ruskinian Gothic building was designed by the architectural firm of Hartwell & Swasey, the precursor to Hartwell & Richardson. Gas lighting, steam heating, up to date plumbing and ventilation systems were included in the construction and concrete was poured between the walls and between the floors to prevent the passage of sound and the spreading of fire. Thayer Academy’s first class was comprised of 26 students.