AMS-27 was laid down as YMS-419 on 17 September 1943 by Henry C. Grebe & Co., Chicago, Illinois; launched 23 March 1944; and commissioned 18 November 1944. Following her shakedown and training period, YMS-419 was assigned to duty in the western Pacific Ocean. She departed San Pedro, California, 27 August 1945 and 27 October began minesweeping operations off occupied Japan. Until the spring of 1946 she swept shipping channels in the Far East, first off Honshū, then between Kyūshū and Korea, and finally around the approaches to the Yangtze River. She then returned to the United States for an extended stay, during which she was redesignated AMS-27 and named Mockingbird 18 February 1947.