Nenonen began his military career in 1894 when he began his studies at the Finnish Cadet School in Hamina. After graduation he continued his studies in Saint Petersburg. He served in the Russian army during World War I. When the Finnish Civil War began he moved to Finland and was given the job of creating the artillery of Mannerheims White Army. After the war he also served as the minister of defence between 1923 and 1924. During the Continuation War he was part of Mannerheims headquarters inner circle. He was promoted to general of the artillery in 1941.
Nenonen developed the Finnish Armys artillery which was to be decisive in the defencive victories of the battles of Tali and Ihantala.
He received the Mannerheim Cross in 1945.