Served on board the USS Lackawanna during successful attacks against Fort Morgan, rebel gunboats and the ram Tennessee, in Mobile Bay, 5 August 1864. Displaying great coolness and exemplary behavior as first sponger of a gun, Cassidy, by his coolness under fire, received the applause of his officers and the guncrew throughout the action which resulted in the capture of the prize ram Tennessee and in the destruction of batteries at Fort Morgan.