In 1868, appointed to Okurairi Bugyo ken Horoyaku (magistrate of directly controlled land and officer of horo (a military kit)), Mondo was assigned to Koidezima in Echigo, which was a exclave of Aizu Domain, to defend Mikuni Pass, but after his younger brother Hisayoshi was killed in the Battle of April 24 he went back to Koidezima, and being further attacked, he moved to the battle line of Echigo.
慶応4年（1868年）御蔵入奉行兼幌役を命ぜられた主水は、会津藩飛領である越後の小出島に赴任し、三国峠を守備するも4月24日の戦いで実弟久吉を戦死させたのち小出島まで撤退、さらに攻められて越後戦線へ移動した。