During World War I, he fought in major offensives, including the Battle of Belleau Wood, and was awarded the Silver Star and the Purple Heart.

Sailing for France in September 1917, Thomas saw action with the 6th Marines at Belleau Wood, Soissons, and in the Meuse–Argonne offensive (Champagne).

In November 1919, he joined the 1st Provisional Marine Brigade in Haiti, taking part in action against Haitian insurgent forces until May 1921.

He returned to the United States in June 1931, entering the Army Infantry School at Fort Benning, Georgia, the following September.

In May 1941, Thomas was transferred to Cairo, Egypt, as a naval observer, but he was recalled to Quantico two months later to become assistant operations officer of the 1st Marine Division.

After that unit was disbanded in March 1949, he became chief of staff of the Marine Corps Equipment Board at Quantico, later serving there as commanding general of the Landing Force Development Center.

He served in that capacity until January 1952, when he was relieved by Major General John T. Selden and returned to the United States.

The following month he was promoted to the rank of lieutenant general and designated by President Truman as assistant commandant of the Marine Corps.

From 1957 to 1958, he was the first director of the National Security Council (NSC) Net Evaluation Subcommittee (NESC); succeeded by Thomas Hickey.