He was born in Litchfield, Connecticut, the first child of a large family. His brother Ira figured prominently in the early history of Vermont. Ethan Allen was the leader of a rebellious group of land owners/speculators who held New Hampshire title to land grants in the Hampshire Grants. New York, which held substantial claim to the grants refused to honor or acknowledge the New Hampshire titles and sold competing titles to different people -- who generally did not live in Vermont. Not surprisingly, this led to open rebellion among the population in much of Vermont.
In the Spring of 1775, Allen and Benedict Arnold led a raid against Fort Ticonderoga. The relative roles of Allen and Arnold are not entirely clear. Neither is it clear to what extent the campaign was formulated by the strongly anti-British faction in Connecticut; to what extent it was the idea of the Green Mountain Boys headquartered at the Catamount Tavern in Bennington; and how much of the enthusiasm was fueled by alcohol rather than by patriotism. What is clear is that the rebels moved North, managed to get a few dozen men across Lake Champlain (they had considerable trouble finding a boat and the one they found was quite small). In a dawn attack, Ticonderoga was taken from the 22 British troops that held it and who were not aware that a war was in progress. Allen/Arnold's rebels also quickly captured forts at Crown Point, Fort Ann on Isle La Motte near the present Canadian border, and (temporarily) the town of St John (now St Jean) Quebec. The comic opera aspects of this campaign notwithstanding, the huge stores of cannon and powder seized at Ticonderoga allowed the American rebels to put in place an effective siege of Boston which caused the British to evacuate in October of 1775.
The Green Mountain Boys elected Allen's cousin Seth Warner as leader, however, Allen commanded a small military force in the American rebel's campaign in Quebec in 1775. As a result of miscommunication or misjudgement he attacked Montreal with a handful of men and was captured by the British. He was shipped to England where he suffered considerable mistreatment. He was later transferred to New York where he was eventually paroled in a prisoner exchange.
Allen then moved back to Vermont which had become a hotbed of anti-everyone sentiment harboring little affection for either England or for the nascent United States and harboring a significant number of deserters from the armies of both. Allen settled a homestead in the delta of the Winooski river near the modern city of Burlington. Allen remained active in Vermont politics and was appointed general in the Army of the independent state of Vermont. He was one of the participants in a failed attempt to bring Vermont back into the British Empire and thereby separate Vermont from New York permanently. Allen's first wife died in 1783 and he remarried in that year. Allen died in 1789 of a stroke at the age of 51.