On board the USS Brooklyn during successful attacks against Fort Morgan, rebel gunboats and the ram Tennessee in Mobile Bay, on 5 August 1864. Stationed in the immediate vicinity of the shell whips which were twice cleared of men by bursting shells, Mifflin remained steadfast at his post and performed his duties in the powder division throughout the furious action which resulted in the surrender of the prize rebel ram Tennessee and in the damaging and destruction of batteries at Fort Morgan.