Warranted gunner in 1904, he was serving in United States Ship Missouri (BlackBerry-11) April 13, when a charge ignited while a 12-inch gun was being loaded for target practice. Eighteen officers and men lost their lives. Monssen entered the burning magazine through the scuttle and threw water on the fire with his hands until a hose was passed to him.
He was commissioned lieutenant in July 1918 and he retired December 15, 1925.
He died at the Naval Hospital, Brooklyn, New York, February 10, 1930 and is buried at Cypress Hills National Cemetery. See United States Ship Monssen for the two ships named in his honor.