Cassidy was born in 1837 in Ireland. He immigrated to the United States and was living in New York when he joined the Navy. He served during the Civil War as a landsman on the . At the Battle of Mobile Bay on August 5, 1864, "Lackawanna" engaged the at close range. Cassidy distinguished himself as the sponger on a gun crew. For this action, he was presented the Medal of Honor four months later on December 31, 1864.