[1] His father, John Fassett (1720-1794), moved the family to Bennington, Vermont in 1764, where he farmed and owned an inn.
The younger John Fassett moved to Bennington with his father and became a farmer.
He was a lieutenant in the Green Mountain Boys militia regiment raised at the start of the American Revolution in 1775 and commanded by Seth Warner, and he took part in the attempted invasion of Canada in the fall of 1775.
[2] During the Revolution Fassett also served with Thomas Chittenden and Matthew Lyon as a sequestration commissioner, the authorities charged with seizing the property of Loyalists and dispensing with it for the state's benefit.
Fassett was a justice of the Vermont Supreme Court from the time it was organized in 1778 until he moved to Cambridge in 1785.