LeDuc served in the American Civil War beginning in 1862, when he was made a lieutenant in the Quartermaster Corps. He aided George McClellan during the peninsular campaign. His service in building corduroy roads in the retreat before the Seven Days Battles was credited by the Minnesota Historical Society as having helped save "a considerable portion of the artillery". LeDuc continued to serve in the Army of the Potomac until after the Battle of Gettysburg, when he was transferred to the Western Army. In the Western Army, LeDuc was involved in the Chattanooga campaign, initially in command of the supplies at Bridgeport, Alabama. However, he soon realized that it would be difficult to efficiently transport supplies and organized the construction of a steamboat to transport supplies up the Tennessee River.