Born in Wetumpka, Alabama, McMorris attended public schools in Wetumpka for several years before entering the United States Naval Academy on June 26, 1908. After graduating fifth in his class on June 8, 1912, McMorris served as an ensign aboard several battleships including the , , and , later taking part in the occupation of Veracruz in 1914 as part of the United States' intervention into Mexico. He also served aboard the battleship , which salvaged the submarine following its sinking off the coast of Honolulu, Hawaii in 1915.