USS Manayunk, a 2100-ton Canonicus class monitor, was built at Pittsburgh, Pennsylvania. Launched in December 1864, she was incomplete at the end of the Civil War and was delivered to the Navy in September 1865 while still not fully outfitted. The ship was laid up at the Mound City Naval Station, Illinois, until 1867 and thereafter at New Orleans, Louisiana. Her name was changed to Ajax in June 1869.

Ajax was finally placed in commission at the beginning of 1871 and operated for several months out of Key West, Florida. She then went to the League Island Navy Yard, at Philadelphia, Pennsylvania, where she was again placed "in ordinary" in July 1871. The monitor had further active service beginning in January 1874, based initially at Key West, then at Port Royal, South Carolina, and in the James River, Virginia. After being laid up at Richmond, Virginia, Ajax was loaned to the New Jersey Naval Militia between 1895 and 1898. In July 1898 she was recommissioned for Spanish-American War duty at Baltimore, Maryland. Decommissioned in September 1898, USS Ajax was sold in October 1899.