(Mon: dp. 2,100; l. 190'; b. 37'8"; dr. 11'10"; s. 6.3 k. a. 2 15"; cl. Canonicus)
Lake in New York, 50 miles south by east of New York City.
The first Mahopac, a heavily armored monitor built by Z. & F. Secor at New York, was launched 17 May 1864, and, after a trial trip 20 August, commissioned at New York, Comdr. William A. Parker in command.
The single‑turreted monitor participated in the joint attack on Charleston 9 September. She operated on the James River in the autumn and steamed to the Wilmington area in time to join the bombardment of Fort Fisher 24 and 25 December 1864 and 13 through 15 January 1865. In the latter attacks, which doomed Wilmington, she was hit eight times without serious damage.
Next steaming to Charleston, Mahopac remained there on picket duty until 8 March when she returned to Chesapeake Bay. In April, she and sister Union ships helped General Grant's force in the capture of Richmond. She decommissioned in June and laid up at the Washington Navy Yard.
Mahopac recommissioned 15 January 1866 and operated along the east coast. Renamed Castor 15 June 1869, she resumed the name Mahopac 10 August. Going into reserve at Hampton Roads 11 March 1872, she recommissioned 21 November 1873 and sailed to Key West for fleet drill and remained there over 2 years. In 1876 she steamed to Port Royal and moved to Norfolk in July 1877. She was stationed with several other monitors at Brandon LandIng until March 1880, when she proceeded to City Point, where she was based until 1888. She was in ordinary at Richmond 1889‑95, then moved to League Island. She was struck from the Navy Register 14 January 1902 and was sold 25 March 1902.