Arthur St. Clair was born in Thurso, Scotland, on 23 March 1736 (Old Style). After a brief stint at the University of Edinburgh, he purchased a commission as an ensign in Great Britain’s 60th Royal American Regiment. He served in Canada under Generals Jeffrey Amherst and James Wolfe, from 1757 to 1759. In 1760, he married Phoebe Bayard. Two years later, he left the British Army and settled in Pennsylvania.
In 1770, he was appointed surveyor of the Cumberland District and then served as the agent of the colonial government in that District. In 1775, he became a colonel in the Pennsylvania militia; a year later he joined the Continental Army at that same rank. By the end of 1776, he was a brigadier general.
St. Clair fought in the early Revolutionary War battles of Trenton and Princeton. In 1777, he was promoted to major general. In July, he abandoned Fort Ticonderoga and faced a court-martial. He was exonerated from blame, and went on to fight at Yorktown and with General Nathanael Greene in the South. In 1783, he left military life to serve in the Continental Congress.
In 1787, St. Clair became the governor of the Northwest Territory, a position he held until 1802. He had subsequently returned to active duty in the Army, and, from 4 March 1791 until 5 March 1792, he served as the senior officer. He was soundly defeated on the Wabash River in November 1791 by Indians, resigned his commission shortly thereafter and resumed the territorial governorship. A panel of inquiry exonerated him from blame, but he was soon removed as governor. St. Clair died in Pennsylvania, on 31 August 1818.