アッツ島沖海戦
Appointed war plans officer to the United States Pacific Fleet following the attack on Pearl Harbor, McMorris remained in this post until April 1942 when he was assigned command of Cruiser Task Force 8 led by the cruiser . After engaging the Imperial Japanese Navy several times, both in the Aleutian Islands Campaign, specifically at the Battle of the Komandorski Islands and (as well as winning distinction for bravery and the award of the Navy Cross during the Battle of Cape Esperance on October 11–12, 1942), McMorris was named Chief of Staff of the Pacific Fleet in June 1943, as well as a personal advisor to Admiral Chester Nimitz, a post in which he would remain until the end of the war.