Located in historic Braintree, Massachusetts, Thayer Academy was founded in 1877, in accordance with the will of General Sylvanus Thayer, a native of Braintree. Sylvanus Thayer led a distinguished life as a soldier, engineer, educator, and philanthropist. He graduated from Dartmouth College as valedictorian of his class in 1807. The following year, he graduated from the U.S. Military Academy at West Point, again at the head of his class, with the rank of Second Lieutenant of Engineers. After several years as a Army engineer, Sylvanus Thayer returned to West Point, becoming the youngest (and longest serving) Superintendent of the U. S. Military Academy.

In addition to founding the Thayer School of Engineering at Dartmouth College, he contributed funds to establish the Thayer Public Library in Braintree, and stipulated in his will that the bulk of of his estate should be used for "the erection of an academy in which young persons of the male sex (or both male and female if my trustees deem it expedient) shall be educated." He proposed "to promote the cause of education in the Commonwealth according to my ability, and benefiting the town of Braintree, the place of my birth."