At the start of the Spanish–American War, at the age of 22, McMurtry left Harvard to serve as a member of Theodore Roosevelt's 1st US Volunteer Cavalry, known as the Rough Riders.

[3] Citation: The President of the United States of America, in the name of Congress, takes pleasure in presenting the Medal of Honor to Captain (Infantry) George G. McMurtry, United States Army, for extraordinary heroism on October 2–8, 1918, while serving with 308th Infantry, 77th Division, in action at Charlevaux, Argonne Forest.

Captain McMurtry commanded a battalion which was cut off and surrounded by the enemy and although wounded in the knee by shrapnel on 4 October and suffering great pain, he continued throughout the entire period to encourage his officers and men with a resistless optimism that contributed largely toward preventing panic and disorder among the troops, who were without food, cut off from communication with our lines.

He continued to direct and command his troops, refusing relief, and personally led his men out of the position after assistance arrived before permitting himself to be taken to the hospital on 8 October.

During this period the successful defense of the position was due largely to his efforts.McMurtry married Mabel C. Post on Long Island on December 16, 1903.