Underhill commanded Preliminary Landing Group during the landing at Kwajalein Atoll. He landed with 25th Marine Regiment and coordinated the landings with artillery support. Underhill was succeeded in his capacity of assistant division commander by Samuel C. Cumming on 11 April 1944. He was promoted to the rank of major general in May 1944 and also decorated with the Legion of Merit for his efforts during the Kwajalein operation.