Jameson was elected June 13, 1776, by the Virginia Convention, captain of the Third Troop of Horse. He had six competitors for the position, and received forty-eight vote, while his competitors received respectively 17, 15, 9, 4, 3, 2 votes. One of his competitors was Henry Lee. Jameson took command June 16, 1776 as captain in a Virginia regiment of dragoons; promoted March 31, 1777, major 1st Continental Light Dragoons, and transferred April 7, 1777, to 3rd Continental Light Dragoons. He fought at the Battle of Brandywine. While staying with George Washington at Valley Forge, Major Jameson was wounded in a skirmish nearby on January 21, 1778. Throughout that year and the next he remained at Washington's side, engaged at the Battle of Monmouth, New Jersey in June, and promoted to Colonel in August 1779.
