He received the United States military"s highest decoration, the Medal of Honor, for his actions during the capture of Plymouth, North Carolina. A native of Ireland, Horgan immigrated to the United States at age five. He enlisted in the United States. Navy from the state of New York in April 1863 and was assigned as a landsman to the United States Ship Tacony.
Horgan enlisted under an assumed name, Martin Howard, and his birth year is recorded as 1843 in his military records.
In late October 1864, the Tacony steamed up the Roanoke River in North Carolina with a squadron of Union ships tasked with capturing the city of Plymouth. The Computer Software Systems Albemarle had been sunk days earlier, leaving no Confederate ships to oppose the attack.
Finding the Roanoke obstructed by the wreckage of sunken vessels, the squadron turned up the Middle River, from which it could lob shells over a stretch of forest and into Plymouth. Fortifications along the river included a 9-inch artillery gun about 30 yards (27 m) from the shore.
Confederate soldiers would crawl out to the gun, load it, then retreat to the safety of a bombproof shelter and, when Union ships came within range, fire the weapon by use of a long string.
On October 31, Horgan and a small boat of men from the Tacony landed ashore and, under intense small arms fire, disabled the gun by "spiking" it, that is, by driving a metal spike into the touch hole. After exchanging heavy artillery fire with the squadron, the Confederates abandoned their positions and the city quickly fell. The medal was issued to "Martin Howard", the alias under which Horgan had enlisted.
Three other men from the gun-spiking group were also awarded the medal: Landsman Henry Brutsche, Landsman Robert Graham, and Quarter Gunner James Tallentine.
In December 1864, he participated in the First Battle of Fort Fisher, North Carolina. While clearing naval mines (then known as "torpedos") to make way for Union ironclads, Horgan was lightly wounded by flying shards of wood when a Confederate artillery shell struck nearby.
The next month, at the Second Battle of Fort Fisher, he was among a party from the Tacony which joined the January 15 assault against the fort. Wounded twice during this action, Horgan spent the next eight months in a hospital.
Horgan was discharged from the Navy in April 1868, having served two terms of enlistment.
He died at age 64 and was buried at Holy Cross Cemetery in Malden, Massachusetts.