He was commissioned a lieutenant in the Royal American Regiment of Foot also known as the 62nd (Royal American) Regiment and the Kings Royal Rifle Corps, on Feb. 23, 1756. The creation of the regiment by Parliament was unique in that it allowed for "a certain number of foreign Protestants, who have served abroad as officers or engineers, to act and rank as officers or engineers in America only..." About fifty officers' commissions were given to Germans and Swiss. Weissenfels was one of these. In 1757, the unit was renumbered as the 60th (Royal American) Foot. Weissenfels served in this regiment for four years. He fought in the French and Indian War during the battles of the Plains of Abraham, Ticonderoga and Havana.