On October 29, 1180, Tomomitsu's mother, Samukawa no Ama, introduced him to Yoritomo who had raised army to defeat the Taira clan; he became Yoritomo's vassal and Yoritomo took the roll of his eboshioya (a person who puts an eboshi [formal headwear for court nobles] on a young man's head on his genpuku ceremony) when he celebrated his genpuku (to become an adult man).
治承4年（1180年）10月2日、平氏打倒に挙兵した頼朝に母寒河尼の引き合わせで臣従し、頼朝が烏帽子親となって元服する。