Before the 4th Marine Division was activated, Horner joined East Coast Echelon of the division at Camp Lejeune in May 1943. He served as chief of staff of the echelon, and, following the absorption of the East Coast Echelon into 4th Marine Division under Major General Harry Schmidt in August 1943, Horner was appointed division infantry operations officer. He sailed for the Pacific theater in January 1944 and subsequently participated in the Battle of Roi-Namur. Horner served on the staff of the 4th Marine Division during the Marianas Islands Campaign in July 1944 and subsequently received the Legion of Merit with Combat "V" for his service as assistant operations officer.