McMorris then held a variety of assignments, including instructor in seamanship and history at the Naval Academy (1925–1927, 1930–1933). In the interval, he commanded destroyers. From 1933 to 1935, he was a navigator on the battleship California. McMorris graduated from the Naval War College in 1938, served as operations officer for the Scouting Force (1938–1939), and then served in the same capacity with the Hawaiian Department (1939–1941).
In January 1941, promoted to captain, McMorris became war plans officer on the staff of Admiral Husband Edward Kimmel, commander in chief of the Pacific Fleet. In April 1942, he took command of the heavy cruiser San Francisco. Promoted to rear admiral, he assumed command of Task Force 8 that December and provided gunfire support for the 7th Infantry Division as it retook Attu Island in the Aleutians. On 26 March 1943, some 200 miles west of Attu and without air support, he fought the Battle of the Komandorski Islands, repelling a stronger Japanese naval force under Vice Admiral Hosogaya Boshiro.
In June 1943, McMorris became chief of staff to Admiral Chester W. Nimitz, commander in chief of the Pacific Fleet and Pacific Ocean Area. He helped plan the Central Pacific offensives of the war's final two years and remained in his post until February 1946. Promoted to temporary vice admiral in September 1944, he took command of the Fourth Fleet in 1946. He served on the General Board from 1947 to 1948. In June 1948, he became a permanent vice admiral, commanding the 14th Naval District and Hawaiian Sea Frontier until he retired in September 1952. McMorris died in Honolulu, Hawaii, on 11 February 1954.
Hoyt, Edwin P. How They Won the War in the Pacific: Nimitz and His Admirals. New York: Weybright and Talley, 1970.; Morison, Samuel Eliot. History of United States Naval Operations in World War II. Vol. 5, The Struggle for Guadalcanal, August 1942–February 1943. Boston: Little, Brown, 1949.