On 6 April 1917, "Chicago" was placed in full commission at Philadelphia and reported to Submarine Force, Atlantic (COMSUBLANT) as flagship at New London, Connecticut, commanded by future Admiral Thomas C. Hart. On 10 July 1919, she departed New York to join Cruiser Division 2 (CruDiv 2), as flagship in the Pacific. She was reclassified CA-14 in 1920 and then CL-14 in 1921. From December 1919 – September 1923, she served with SubDiv 14 and as tender at the Submarine Base, Pearl Harbor.