Born in 1856 in Ireland, Thornton immigrated to the United States and joined the Navy from Pennsylvania.
By August 26, 1881, he was serving as a seaman on the tugboat USS Leyden.
On that day, while Leyden was near Boston, Massachusetts, Landsman Michael Drennan jumped overboard because he was "temporarily insane".
For this action, he was awarded the Medal of Honor three years later, on October 18, 1884.
[1] Thornton's official Medal of Honor citation reads: For jumping overboard from the U.S. Tug Leyden, near Boston, Mass., 26 August 1881, and sustaining until picked up, Michael Drennan, landsman, who had jumped overboard while temporarily insane.