A junior officer during World War I, McMorris saw combat in the Atlantic aboard the destroyers Shaw (DD-68) and Meredith (DD-165) prior to his promotion to lieutenant in 1918.

During the interwar years, he was stationed in various sea and shore posts before his graduation from the Naval War College in 1938, serving as operations officer to the Hawaiian-based US fleet from 1939 until 1941.

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 Task Force 8 led by the cruiser San Francisco (CA-38).

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),[1] 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 he retained until the end of the war.

He eventually commanded the Pearl Harbor naval base before retiring to Marietta, Pennsylvania, where he lived until his death in 1954.