His father, William Allen, soon after the start of the American Revolution in June 1775, was commissioned an ensign in the 3rd Rhode Island Regiment in the Continental Army. His regiment served in the Siege of Boston and was disbanded on December 31, 1775. In January 1776, he was promoted to 1st lieutenant in the 11th Continental Infantry Regiment and served until the regiment was disbanded at the end of the year. In January 1777 he was promoted to captain in the 2nd Rhode Island Regiment. He commanded a company of the Rhode Island line of troops at the Battle of Saratoga. He transferred to the 1st Rhode Island Regiment when the two Rhode Island regiments were consolidated on January 1, 1781. William Allen was present and actively engaged in many of the battles which were fought during our Revolutionary War. In recognition of his service, he was advanced to the brevet (honorary) rank of major on September 30, 1783. He served until the Continental Army was disbanded on November 3, 1783. In 1786, Allen was appointed by Congress, senior officer of the legionary corps raised in Rhode Island. In 1799, he was appointed by the Rhode Island General Assembly a brigadier general of militia.