Magpie was laid down as YMS-400 by Henry B. Nevins, Inc., City Island, New York, New York, 3 July 1942; launched 24 March 1943; sponsored by Mrs. Marie Norby; and commissioned 15 May 1943, Lt. Alan G. Lynn, USNR, in command. The new auxiliary motor minesweeper departed Staten Island for Norfolk, Virginia, 1 June, via the Chesapeake Bay; served briefly at Yorktown, Virginia; and escorted three merchant ships from Norfolk to Miami, Florida, arriving 27 June. She continued on to Key West, Florida, arriving the 30th. Magpie was decommissioned 20 October 1950.