After the assassination of II, Yoshinobu HITOTSUBASHI (Yoshinobu TOKUGAWA), who had become the Shogun's successor with the support of the Imperial Court and the Satsuma Clan, started the Bunkyu Reformation along with Yoshinaga MATSUDAIRA, the head of the Fukui Clan who had become Seiji Sosaishoku; and in 1862 Katamori was appointed to the new post of Bakusei Sanyo, and later to the newly created post of Kyoto Shugoshoku.
井伊暗殺後に朝廷や薩摩藩の後援で将軍後継となった一橋慶喜（徳川慶喜）、政事総裁職となった福井藩主・松平慶永らが文久の改革を開始すると、文久2年（1862年）に新設の幕政参与に任ぜられ、のち新設の京都守護職に推される。