Cornplanter, whose Seneca name is rendered Gaiantwaka in the historical record, was born around 1750 at Canawaugus, a Seneca village on the Genesee River, the son of a Seneca mother and a Dutch trader named John Abeel. He came to military prominence among the Seneca during the American Revolution, leading war parties in the Battle of Oriskany, the Battle of Wyoming, the Cherry Valley raid of November 1778 and the fighting against General Sullivan's 1779 campaign into Haudenosaunee territory, continuing raids into 1780.
After the Revolution, Cornplanter turned from war leadership to treaty diplomacy with the young United States, becoming one of the most prominent Seneca voices at the negotiating table. He was a principal Seneca signatory of the Treaty of Fort Stanwix in 1784, the Treaty of Canandaigua in 1794 and the Treaty of Big Tree in 1797, agreements that ceded large tracts of Haudenosaunee land and drew sharp criticism from other Seneca leaders, most notably Red Jacket, who argued Cornplanter had conceded too much. In 1796 the Commonwealth of Pennsylvania granted him about fifteen hundred acres of former Seneca territory along the Allegheny River for himself and his heirs; the tract took his name and, by 1798, was home to some four hundred Seneca.
Cornplanter used his standing to court Quaker educators, inviting them to establish schools on his land to teach practical skills, a choice that put him at the center of debate over how far the Seneca should accommodate the encroaching United States. His brother Handsome Lake's religious reform later turned that debate toward the tradition his own diplomacy had helped his people survive within.