Colonel Underhill was transferred to the Headquarters Marine Corps in Washington, D.C., where he was appointed executive officer of the Adjutant and Inspector's Department in July 1939.

He served in this capacity until March 1942, when he was appointed commander of the Marine Corps Base San Diego, California.

[2] One year later, Brigadier General Underhill was transferred to the Command of Marine Corps Base Camp Lejeune, North Carolina.

He subsequently supervised the training together with the division commander, Major General Harry Schmidt, before it was shipped out to the Pacific in January 1944.

[2] Underhill returned to the United States and was appointed president of the Postwar Personnel Reorganization Board at Headquarters Marine Corps, Washington, D.C. in 1946.