In 1180, when MINAMOTO no Yoritomo who had been also exiled to Izu Province raised an army to fight against Heike (the Taira family), Mochimitsu joined him and fought against Kagechika OBA and Sukechika ITO who supported the Taira family in the Battle of Ishibashiyama.
治承4年（1180年）、同じく伊豆国において流人生活を送っていた源頼朝が反平家の兵を挙げるとその足下に駆けつけ、石橋山の戦いで平家方の大庭景親・伊東祐親らと交戦。