Warranted gunner in 1904, he was serving in Missouri when, on 13 April, a charge for a 12-inch gun ignited during target practice, killing eighteen. Monssen entered the burning magazine and threw water on the fire with his hands until a hose was passed to him, for which he was awarded the Medal of Honor.
Commissioned lieutenant in July 1918, he retired in 1925 and died 10 February 1930 at Brooklyn Naval Hospital.
Source: Dictionary of American Naval Fighting Ships, Naval Historical Center.