He served on the Mexico–United States border during the Pancho Villa Expedition, and in 1917 completed officer training and received appointment as a second lieutenant of Field Artillery.

During World War I and the post-war American Forces in Germany, Whitlock managed transportation at the Brest, France, port of embarkation.

[3] After completing his education, Whitlock worked for Goodyear Tire and Rubber in Akron[4] and joined the Ohio National Guard's Battery B, 1st Field Artillery Regiment with the intention of obtaining a commission in the United States Army.

[5] In July 1917, Whitlock completed the Citizens' Military Training Camp held at Fort Sheridan, Illinois, and applied for an army commission.

[5] Whitlock was assigned to the 2nd Field Artillery Regiment for World War I and completed training with his unit at Camp Fremont, California.

[9] The division did not deploy to France as a unit; some components went overseas as part of American Expeditionary Force, Siberia and some were recalled before arriving in Europe and demobilized.

[9] The 2nd Field Artillery was part of an 8th Division contingent that went to France in late summer 1918, where they constructed and managed the Allied port of embarkation in Brest.

[9] Whitlock remained in France after the war as railway transportation officer for soldiers assigned to American Forces in Germany, and assisted them in returning to the United States for demobilization.

[12] He graduated from the course in 1923,[5] after which he was assigned as assistant professor of military science for the Reserve Officers' Training Corps at the University of Oklahoma.

[17] In the fall of 1933, Whitlock planned and oversaw construction of CCC camps at Papago Park and Tonto National Forest in Arizona.

[22] As MacArthur and his staff planned the New Guinea Campaign and follow on operations against the Japanese, Whitlock was commended for making use of assistance from individuals with expertise in oil production, shipping, and other areas of importance.

[28] When the Japanese signed the Instrument of Surrender aboard USS Missouri on September 2, Whitlock was among those on hand to witness the ceremony.

[30] In August 1947, Whitlock returned to the United States and assumed command of the San Antonio General Depot in Texas.

[32] In August 1948, the 10th Mountain Division was reactivated to provide basic training to draftees at Fort Riley, Kansas, and Whitlock was named to command it.