Monroe served in the Virginia regiment and later in the Continental Army. The Third Virginia Infantry later met up with General George Washington. Monroe was serving under Washington during the Battle of Trenton. In this battle, Monroe suffered a gunshot wound after leading the troops into battle. Fortunately, a Dr. Riker saved Monroe and stopped the severe bleeding. After his bravery at the battle, Monroe was promoted to the rank of captain. He had to take three months off to heal.

He returned to camp as a major, under the command of William Alexander, a brigade commander for George Washington. Monroe served during the rough winter of Valley Forge. For his service, Washington sent a positive letter of recommendation to Virginia officials, who named Monroe a lieutenant-colonel in the Virginia Regiment. However, he had no cadre of soldiers to lead, and later transitioned to a career in politics.

Monroe remained very proud of his military service throughout his life. Even while he was president, he preferred the title “colonel.” He even donned a blue tunic like the one worn by General Washington during the time of the Revolutionary War.