Between the wars, Holcomb spent further tours in Germany, Cuba, and Beijing. He graduated from the Fort Leavenworth Army Staff College, the Naval War College, and the Army War College and served in the Office of Naval Operations, winning promotion to brigadier general in 1935. The following year, President Franklin D. Roosevelt appointed him major general and commandant of the Marine Corps, advancing him above more senior officers.
As commandant, Holcomb undertook initiatives that would bear fruit during the Pacific war. He greatly enhanced the Marines' growing capacity for amphibious warfare, developing several new types of landing craft. He supervised the expansion of the Marine Corps from 16,000 officers and men in 1936 to 50,000 in mid-1941, 143,000 six months later, and more than 300,000 by 1945, comprising six combat divisions and four air wings. In December 1940, Roosevelt appointed Holcomb to a second term as commandant, and in February 1942, Holcomb was promoted to lieutenant general. Holcomb trained and equipped the Marines but had no operational control over their activities in the field. After disputes in 1942 at Guadalcanal between Rear Admiral Richmond Kelly Turner and Marine commander Major General Alexander Vandegrift, Holcomb insisted on the definition of clear lines of authority in joint amphibious operations. He won the decision that navy and Marine Corps commanders would enjoy equal authority and that superior officers would resolve any disputes. As commandant, he also resisted racial integration of the Marines on the grounds that it would detract from combat efficiency.
Holcomb reached mandatory retirement age in August 1943, but Roosevelt retained him in post and made him the first Marine officer ever to become full general. At the end of 1943, Holcomb insisted on making way for a younger successor and retired. He then spent four years as ambassador to South Africa, seeking modest mitigation of that nation's racial policies. Holcomb returned to the United States in 1948. He died in New Castle, Delaware, on 24 May 1965.
Gordon, John. "General Thomas Holcomb and ‘The Golden Age of Amphibious Warfare.'" Delaware History 21 (1985): 256–270.; Heinl, Robert D., Jr. Soldiers of the Sea: The U.S. Marine Corps, 1775–1962. 2d ed. Baltimore, MD: Nautical and Aviation Publishing Company of America, 1991.; Hough, Frank O., et al. History of U.S. Marine Corps Operations in World War II. Vol. 1, Pearl Harbor to Guadalcanal. Washington, DC: Government Printing Office, 1958.; Millett, Allan R. "Semper Fidelis": The History of the United States Marine Corps. Rev. and expanded ed. New York: Free Press, 1991.; Shaw, Henry. Opening Moves: Marines Gear Up for War. Washington, DC: History and Museums Division, United States Marine Corps, 1991.; Truscott, Lucian K., Jr. The Twilight of the U.S. Cavalry: Life in the Old Army, 1917–1942. Edited and with Preface by Lucian K. Truscott III and Foreword by Edward M. Coffman. Lawrence: University Press of Kansas, 1989.