Sheldon was inducted into the U.S. Army on February 17, 1941 at Marshfield, Massachusetts and processed at Fort Devens, Massachusetts. He was assigned to the 34th Field Artillery Battalion Battery 'B' 9th Infantry Division Fort Bragg, North Carolina. At Fort Bragg Sheldon completed his basic training and participated in the Carolina maneuvers and conducted amphibious landing training on beaches in Virginia. Sheldon received his promotion to First Sergeant on September 4, 1942. The 9th Division moved to Fort Dix, New Jersey on December 1, 1942 for equipment and staging for deployment overseas. His unit boarded the SS (USAT) URUGUAY at Staten Island, New York on December 11, 1942 and sailed for their overseas destination on December 12, 1942.

He received a Bronze Star for leadership in the success of his unit in combat during the period February 22, 1943 to July 16, 1943. Sheldon's Artillery Battalion was one of the first units to receive a Presidential Unit Citation for action against the enemy. During the Battle of El Guattar Sheldon met brother Charles. This was their first meeting since they were stationed at Fort Bragg, North Carolina. In June 1943, with the end of the Tunisian Campaign the 9th Division moved to a bivouac area near Oran, Algeria for rest.

On July 5, 1943 the 9th Division moved to Bizerte, Tunisia. The unit was loaded on LST'S at the port of Bizerte and departed July 14, 1943 for Sicily and landed in Licata, Sicily. On July 15, 1943 the 9th Division unloaded equipment and prepared for combat. In the following months the 9th Division moved north by motor convoy traveling through Caltanissette, Enna, and Micosia, Sicily. In November the 9th Division was bivouacked northwest of Palermo at a staging area. Sheldon met brother Elmer again in this staging area. Elmer informed Sheldon that his unit was moving on to the Italian Campaign.

On November 10, 1943 the 9th Division moved to the port of embarkation at Palermo and loaded equipment and troops on the MS John Erickson and departed for Liverpool, England. The 9th Division arrived in Liverpool, England on November 25, 1943. They disembarked and moved to Barton Stacey Camp, England. For the next six months the 9th Division trained for the invasion of Europe. The troops were given the luxury of furloughs and week-end passes while they completed the training.

By April 1944 all leaves and furloughs were canceled and the training pace was accelerated. On June 6, 1944 the 9th Division moved to the marshaling area at Winchester, England. The 9th Division embarked from Southampton, England on June 8, 1944 for France.

The 9th Infantry Division hit the Normandy Beach on D plus-4 and was one of the two U.S. infantry divisions with previous combat experience. They bivouacked in the vicinity of Fauville, Normandy, France. Moving across France, the 9th Infantry Division was the first allied force to begin the liberation of Belgium when they took up firing positions near Beaumont, Belgium on September 3, 1944. On September 14, 1944 the 9th Infantry Division crossed into Germany at Rotgen. The 34th Field Artillery Battalion, 9th Infantry Division provided fire support during the Ardennes Offensive (Battle of the Bulge). Sheldon was relieved of his First Sergeant position and was assigned the duties of an Artillery Forward Observer. He was appointed Second Lieutenant on January 26, 1945 (Battle Field Commission). The 9th Infantry Division was at the Elbe River in April 1945 and with the link-up with the Russian Army. The war ended for the 9th Infantry Division at Kothen and Dessaw, Germany. Sheldon had another visit with Charles at the end of the war in Munich, Germany. Sheldon departed Germany on September 6, 1945. He was discharged from the Army on October 3, 1945 at Fort Devens, Massachusetts