Jacob Spear was born 8 Oct 1755 in Braintree, Massachusetts, son of Samuel Spear (1716-1776) and Mary Ford (1728-1788).

He was a soldier in the American Revolution, enlisting as a private on 9 Jan 1776 and ultimately attaining the rank of Corporal of Marines in the Navy. As a part of his mother's pension application, their son, also named Jacob, wrote a detailed account of his father's service in the war, including his injury in the Battle of Trenton, and six months as a prisoner of the British, which he escaped in Jan 1778. He re-enlisted in summer 1779, serving on Navy ships and privateers.

After returning from the war, Jacob was active in his community of Braintree, Massachusetts, and also took an interest in the opening of new lands in Vermont. He was one of the 65 petitioners, mostly residents of Braintree, Massachusetts and vicinity, who asked the Vermont legislature to grant a charter for the town of Braintree, Vermont in 1779. This was granted on 2 Nov 1780 and a charter was issued by the Governor of Vermont on 1 Aug 1781. The petitioners became proprietors, with an ownership interest in the lands granted by the legislature.

The proprietors first met as a formal body on 6 Aug 1783 in Brookline, Massachusetts. At that meeting they appointed Jacob Spear and Jonathan Holman to draw up a plan for the town, including the layout of lots and streets. Jacob and Holman submitted their initial plan, which was approved at the next 29 Oct 1783 meeting. The proprietors then formed a larger committee led by Jacob to actually ride to the townsite and mark the corners on the ground. Subsequent plans and surveys added more divisions, each proprietor receiving three lots of 100 acres each. Jacob was one of the people who was employed to construct and maintain the roads..

Jacob was elected one of the three members of the first Board of Selectmen in 7 Apr 1788. Although he was not given the rank of Captain in the war, he was known by that title in civilian life, perhaps in honor of his contribution to the creation of the town.