St. Louis departed Honolulu on 9 April to join the cruiser force engaged in escorting convoys bound for Europe. Calling first at San Diego, she took on board 517 National Naval Volunteers and apprentice seamen to bring her war complement to 823 officers and men. On 20 April, she was placed in full commission. A month later, she arrived in the Panama Canal Zone and embarked the 7th, 17th, 20th, 43d, 51st and 55th companies of Marines. She transported them to Santiago de Cuba and then sailed for Philadelphia, arriving on 29 May 1917. St. Louis's first convoy duty began on 17 June 1917 when she departed New York in escort of Group 4, American Expeditionary Force. Returning to Boston for repairs on 19 July 1917, she had completed six additional voyages, escorting convoys bound from New York for ports in Britain and France by the end of the war. Together with the USS Huntington she left Halifax on 30 October 1917 carrying members of a high-level U.S. Commission to confer with the European Allies. Escorted by USS Balch and other British and American vessels she arrived in Plymouth on the evening of 7 November. Post-war After the Armistice, St. Louis was immediately pressed into service returning troops to the United States. She returned 8,437 troops to Hoboken, New Jersey, from Brest, France, in seven round-trip crossings between 17 December 1918 and 17 July 1919 when she arrived at the Philadelphia Navy Yard for repairs. Designated CA-18 on 17 July 1920 and assigned to post-war duty with the European Squadron, St. Louis departed Philadelphia on 10 September 1920 for Sheerness, Cherbourg and Constantinople. She disembarked military passengers at Sheerness on 26 September, then continued on to the Mediterranean and reported to the Commander, United States Naval Forces in Turkish Waters at Constantinople on 19 October. Standing up the Bosphorus from Constantinople on 13 November, St. Louis embarked refugees at Sevastopol and Yalta, returning them to Constantinople on 16 November. The following day, her crew formed boat landing parties to distribute food among refugees quartered aboard naval transports anchored in the Bosphorus. St. Louis continued her humanitarian duties at Constantinople and at Anatolian ports during the time of unrest caused by the Russian Civil War and the Turkish revolution. She departed Asia Minor for Naples on 19 September 1921 and called at Gibraltar. Decommissioning and fate On 11 November, St. Louis