Mitchell was an officer of the
Continental Army during the
American Revolution. In 1776, he was captain of a Delaware company raised under Colonel Samuel Patterson as part of what was known as the "
Flying Camp". They were stationed at
Perth Amboy, New Jersey, and saw no action. When the "Flying Camp" disbanded, the company was attached to Colonel David Hall's regiment, but fought with Colonel
William Grayson's Virginians at the
Battle of Brandywine. Nursing an illness, he was not at
Germantown, but spent the winter at
Valley Forge. Following Grayson's promotion to brigadier-general, Mitchell led his regiment in the attack at the
Battle of Monmouth. This was the attack that was ordered back by General Charles Lee and which eventually led to his court-martial. In 1779, Mitchell was brigade major on General
Peter Muhlenberg's staff in the tidewater Virginia. When British General
Benedict Arnold attacked
Richmond, Virginia, Mitchell was defending
Petersburg when he was captured on May 10, 1781. By most accounts, his childhood friend Michael O'Brien died in the affray. He was held prisoner until after the
Battle of Yorktown. ==Professional and political career==