The ship was reactivated in 1870, having been renamed Terror the previous year, and was assigned to the North Atlantic Fleet where she served in the Caribbean Sea.

[5] Her main battery consisted of four smoothbore, muzzle-loading, 15-inch (381 mm) Dahlgren guns mounted in two twin-gun turrets, one each fore and aft of the single funnel.

[8] While still building in early 1864, she was modified with the addition of a turret-roof-height "hurricane deck" that stretched between the two turrets and around the funnel and main ventilator to improve her navigational facilities.

[7] Agamenticus was commissioned on 5 May 1865 and was prepared to fight the Confederate ironclad CSS Stonewall that was at sea somewhere in the Atlantic Ocean at that time.

Stonewall went elsewhere[1] and the monitor operated off the northeastern coast of the United States until she was decommissioned at the Boston Navy Yard on 30 September.

At that time it mostly operated in the Caribbean Sea, protecting American citizens and interests during the Ten Years' War in Spanish Cuba and unrest in the West Indies.

[7] The monitor was relieved of her assignment at Key West, Florida, on 17 May 1872 and she was towed to the Philadelphia Navy Yard, Pennsylvania, by the tugboat USS Powhatan.