Larsen joined the United States Marine Corps at the age of 23; he earned top marks in a class of 75 at Officer Candidates School. He commanded the 3rd Battalion 5th Marines during World War I. Shortly after being field promoted to the rank of major, he led a liaison mission on June 25, 1918, during the Battle of Belleau Wood, for which he received a Citation Star. On October 4, 1918, the battalion's flanks became exposed while operating alongside the Fourth French Army near Rheims. The Germans attacked multiple times, but despite this the battalion managed to maintain all of the ground they had taken. For Larsen's heroic command during the two-day defense he was awarded the Navy Cross and a second Citation Star. He received a third Citation Star during the war; all three were later converted to Silver Stars, making him the recipient of three Silver Star awards. He participated in every major action undertaken by the Marine Corps during the First World War, and was the only member of the 3rd Battalion to serve during the war without being wounded. In 1919, France awarded him the Croix de guerre with palm and the Legion of Honour for his actions during the war.