 Assigned to the section patrol in the 1st Naval District in northern New England, "Halcyon" performed harbor patrol duties for the remainder of World War I and in its immediate aftermath.The Navy decommissioned her on 24 June 1919, struck her from the Navy List on 31 July 1919, and transferred her back to the Bureau of Fisheries on 5 August 1919.The Bureau of Fisheries reported that the Navy transferred "Halcyon" back in good condition, but also that she was not fully complete or equipped at the time of her 1917 transfer to the Navy and that she needed additional work before she was ready for BOF service.