When the U.S. entered World War I in 1917, Darrah was senior instructor at the Officers' Training Camp at Fort Benjamin Harrison in Indianapolis.

[1][2][3] After the war, Darrah attended office training schools and then served in various chief-of-staff and command positions.

Darrah returned to the position of chief of staff, this time of the Third Corps Area, until 1931 and then from 1932 to 1934 he commanded the Pacific Section of the Panama Canal Department.

[1] Darrah was awarded with two Silver Star commendations for "gallantry in action" during his services in Santiago, Cuba and Legaspi, Luzon in the Philippines.

[1][3] Although retired from the military, during World War II Darrah served as New York State's Deputy Director of Civilian Defense.