At age twenty, McDowell participated in the French and Indian War. He was captain of a company, serving under George Washington at Braddock’s Defeat at the Battle of the Monongahela. Later, he served in Lord Dunmore’s War, participating in the Battle of Point Pleasant with future Kentucky governor Isaac Shelby. Shelby later appointed McDowell as his aide-de-camp. For his service in the war, he was awarded a large tract of land in Fayette County, Kentucky in 1775.
At the outbreak of the Revolutionary War, McDowell was commissioned a Major over a regiment from Augusta County. His command acted as Rangers scouting the mountain passes for Indian activity. In 1777 McDowell led the Augusta County militia to Fort Pitt to participate in campaign planned by General Hand into the Ohio Country. The campaign was canceled due to lack of supplies. While McDowell was at Fort Pitt he was selected to announce the surrender of Burgoyne at Saratoga to the troops. He was promoted to Colonel of the Rockbridge County Militia (Rockbridge County split off of Augusta County) and participated in the Battle of Guilford Courthouse while serving under General Nathanael Greene. The Rockbridge County men fought on the left flank of the American army and inflicted heavy casualties on the attacking British infantry before being driven off by Banastre Tarlton’s Mounted Legion.