When his older brother MINAMOTO no Yoshiie became hard-pressed during the Gosannen no Eki (The Latter Three Years' War), Yoshimitsu, who was in the capital, renounced his court rank and headed for Oshu (Mutsu province) in order to help.
後三年の役の時に苦戦する兄の源義家を助けるため都での官位を捨てて奥州へ向った。