He was heavily decorated for valor, and is one of the iconic fighting men of the American Expeditionary Forces (AEF) during World War I.

Among his classmates included several general officers of the future, such as Charles Gerhardt, Charles S. Farnsworth, Nathaniel Fish McClure, Michael Joseph Lenihan, Herman Hall, William Weigel, Ernest Hinds, Mark L. Hersey, James Theodore Dean, Frank Herman Albright, Marcus Daniel Cronin, George Owen Squier, Thomas Grafton Hanson, George Washington Gatchell, Alexander Lucian Dade and Edmund Wittenmyer He was posted to Fort Meade, Dakota Territory, and Forts Custer and Missoula, Montana for service in the American Indian Wars.

McAlexander received the Silver Star for "gallantry in action in the Santiago de Cuba Campaign, 22 June to 17 July 1898.

Upon his promotion to major in 1911, Ulysses G. McAlexander was sent to be an active duty trainer and inspector for the Oregon Army National Guard.

When the United States entered the First World War in April 1917, McAlexander was promoted to colonel in May and sent to France to command the 18th Infantry Regiment[5] of the 1st Division.

It is said that McAlexander was relieved from his command position by General R. Bullard for profanely refusing to believe that the French could teach him anything about war.

The French commander, General Jean de Montdésir, wanted to piecemeal the Americans among his troops, but McAlexander would have none of it, and the 38th wasn't budged until the 3rd Division was given a front of its own, in June, near Chateau-Thierry.

[8] Without yielding his hold on the Marne embankment, McAlexander refused both flanks so that his regimental front stood like a horseshoe, one battalion forward, one on either side.

For his decisive action, McAlexander received the Army Distinguished Service Medal, the citation for which reads: The President of the United States of America, authorized by Act of Congress, July 9, 1918, takes pleasure in presenting the Army Distinguished Service Medal to Brigadier General Ulysses Grant McAlexander, United States Army, for exceptionally meritorious and distinguished services to the Government of the United States, in a duty of great responsibility during World War I.

General McAlexander commanded the 38th Infantry Regiment, 3d Division, with marked distinction in repelling the German attack at Mezy, south of the Marne, in July 1918.

Jesse W. Woolridge, he enjoyed his finest five minutes of the war believing he was the spear of the assault, when he bumped into McAlexander, crawling forward at the very front of his regiment.

The citation for the medal reads: The President of the United States of America, authorized by Act of Congress, July 9, 1918, takes pleasure in presenting the Distinguished Service Cross to Brigadier General Ulysses Grant McAlexander, United States Army, for extraordinary heroism in action while Commanding the 38th Infantry Regiment, 3d Division, A.E.F., near Jaulgonne, France, 22 July 1918.

General McAlexander, then a colonel, commanding the 38th infantry, displayed exceptional gallantry when his regiment, attacking without support on either flank, was stopped by severe machine-gun and artillery fire, by going ahead of the most advanced elements of his command, and in full view of the enemy, leading his men by force of his own example to the successful assault of Jaulgonne and the adjoining heights.

Later in the day, when progress was again checked, he personally reconnoitered to within 50 yards of hostile machine-gun nests, and through information thus obtained, was enabled to hold an advanced position, with both flanks exposed, for more than 36 hours.

When the Great War ended on 11 November 1918, McAlexander stayed with the Allied occupation force in Germany until 1919, when he returned home.