Mahaska was built at the Portsmouth Navy Yard, Kittery, Maine, for $130,001.68; launched 10 December 1861 and commissioned 5 May 1862, Lt. Norman H. Farquhar in command. Mahaska sailed from Portsmouth 15 May 1862, reporting shortly thereafter for duty in the rivers flowing into Chesapeake Bay. On 20 June she engaged the Confederate batteries along the Appomattox River and on 1 November destroyed entrenchments at West Point, Virginia. Continuing her patrols into the next year, she captured the schooner General Taylor in Chesapeake Bay 20 February 1863. Moving south later in the year, she joined in the blockade of Charleston and participated in the attacks on the forts and batteries in that harbor: Fort Wagner, 8 and 18 August; Morris Island, 13 to 17, and 20 August; and Fort Sumter, 21 Augu