Built as American steam merchant Cornish for Eastern SS Lines Inc, Boston MA and served on the Great Lakes. On 29 Dec, 1941 acquired by the US Navy and converted to the auxiliary patrol yacht USS YAG-9 by the Sullivan Drydock & Repair Co, New York. The vessel was armed with one 4in, one 3in and four 20mm guns and two depth charge tracks and commissioned on 3 Jan, 1942.
She was assigned to the Third Naval District and used for patrol duty off New York until renamed and reclassified USS Muskeget (AG 48) on 30 May 1942.

On 1 Jul, 1942, transferred to the US Coast Guard, reclassified as USS Muskeget (WAG 48) and assigned to the station at Boston for duty as weather ship in the Weather Observation Patrol. The ship then patrolled on Weather Station No. 2 (53°N/42°30W) from 6 to 27 July. 

On 24 Aug, 1942, the USS Muskeget (WAG 48) departed for her second patrol as weather ship on the Weather Station No. 2 (53°N/42°30W) and sent weather reports until 9 September, but then no further messages were received. When the weather ship USS Monomoy (WAG 275) failed to locate her on relief four days later, a combined search by aircraft and ships was carried out on 16 September, but proved fruitless and the ship was reported missing.

At 15.16 hours on 9 Sep, 1942, U-755 fired a spread of three torpedoes at an auxiliary vessel and heard two hits, followed by sinking noises. This must have been the USS Muskeget (WAG 48). All men on board were lost: nine officers, 107 ratings, one Public Health Service officer and four civilian US Weather Service employees.