Masanosuke Subu was born in 1823. He was the son of Kanemasa, a retainer of the Choshu Clan (Yamaguchi Prefecture).
As a leading retainer, he participated in administrative and defense system reforms of the clan (1854). When the Court issued a secret message for an attack on the Tokugawa Shogunate (1858), he conveyed his lord's confidential reply to the Court.
After 1861, he was active in promoting cooperation between the Court and the Shogunate and reviving the Imperial regime. As a member of the Choshu Clan he advocated an antiforeign policy, but when the policy of the Court was reversed on this point at a Court conference (1863), he fell in disfavor.
He later killed himself, holding himself responsible for the deteriorated position of his clan.