Stubby served with the 102nd Infantry, 26th Division in the trenches in France for 18 months and participated in four offensives and 18 battles. The loud noise of the bombs and gun fire did not bother him. He was never content to stay in the trenches but went out and found wounded soldiers.

Stubby entered combat on February 5, 1918 at Chemin Des Dames, north of Soissons, and was under constant fire, day and night for over a month. In April 1918, during a raid to take Schieprey, Stubby was wounded in the foreleg by the retreating Germans throwing hand grenades. He was sent to the rear for convalescence, and as he had done on the front was able to improve morale. When he recovered from his wounds, Stubby returned to the trenches.

After being gassed and nearly dying himself, Stubby learned to warn his unit of poison gas attacks, continued to locate wounded soldiers in no man's land, and since he could hear the whine of incoming artillery shells before humans could, became very adept at letting his unit know when to duck for cover.

He was solely responsible for capturing a German spy in the Argonne. The spy made the mistake of speaking German to him when they were alone. Stubby knew he was no ally and attacked him biting and holding on to him by the seat of his pants until his comrades could secure him.

Following the retaking of Chateau-Thierry by the US, the thankful women of the town made Stubby a chamois coat on which were pinned his many medals. There is also a legend that while in Paris with Corporal Conroy, Stubby saved a young girl from being hit by a car. At the end of the war, Conroy smuggled Stubby home.

After returning home, Stubby became a celebrity and marched in, and normally led, many parades across the country. He met Presidents Woodrow Wilson, Calvin Coolidge, and Warren G. Harding. Starting in 1921, he attended Georgetown University Law Center with Conroy, and became the Georgetown Hoyas' team mascot. He would be given the football at halftime and would nudge the ball around the field to the amusement of the fans.

Stubby was made a life member of the American Legion, the Red Cross, and the YMCA. In 1921, the Humane Education Society awarded him a special gold medal for service to his country. It was presented by General John Pershing.

In 1926, Stubby died in Conroy's arms. His remains are featured in The Price of Freedom: Americans at War exhibit at the Smithsonian. Stubby was honored with a brick in the Walk of Honor at the United States World War I monument, Liberty Memorial, in Kansas City at a ceremony held on Armistice Day, November 11, 2006.