Allen's father, Joseph, intended Ethan to attend Yale University, but his death in 1755 precluded that option. Instead, Ethan, the oldest of seven children, took over the family landholdings. Two years later, Ethan made his first visit to the New Hampshire Grants, land that is now within the state of Vermont, as part of the Litchfield County militia during the Seven Years' War.
Having acquired land in the area, in 1770 Ethan Allen became the colonel-commandant of the Green Mountain Boys, a militia founded in what is now Bennington, Vermont, to defend the New Hampshire Grants. In an inter-colonial fracas, both New Englanders, like Allen, and colonial New Yorkers claimed land in the Green Mountains. Although Allen's vigilantes took no lives, they were willing to use lesser forms of physical intimidation to scare New Yorkers into leaving the area.
Allen and his boys proposed political independence for their district between the Connecticut River and Lake Champlain before the American Revolution caused their attention to shift towards independence from Britain. In 1775, Allen and the Green Mountain Boys captured Fort Ticonderoga from the British in a joint effort with Colonel Benedict Arnold, who had been commissioned by Massachusetts and Connecticut to stage an attack to prevent British forces from marching on Boston. The same force took control of Crown Point, New York, the following day without facing any opposition. The two easy victories garnered for the Patriots much-needed cannon that they then used to drive the British from Boston. Later in the year, the British captured Allen during the botched Patriot attempt to seize Quebec.
In 1777, Vermonters formally declared their independence from Britain and their fellow colonies when they created the Republic of Vermont. After the war concluded, the independent Vermont could not join the new republic as a state, because New York, Massachusetts and Connecticut all claimed the territory as their own. In response, frustrated Vermonters, including Allen, went so far as to negotiate with the Canadian governor, Frederick Haldimand, about possibly rejoining the British empire.
Ethan Allen died on his farm along the Winooski River in the still independent Republic of Vermont on February 12, 1789. Two years later, Vermont finally managed to join the new republic as its 14th state.