In the beginning of the Kyotoku War, Tomomasa was a postulant in Rokuonin of the Shokoku-ji Temple, but later he quit the priesthood for some reason, took part in dominating the territory of the Ogigayatsu Uesugi family, and entered the Nanasawa Fort in the Sagami province (the present Atsugi City, Kanagawa Prefecture).
享徳の乱初期には相国寺鹿苑院の喝食だったが、後に何らかの理由で還俗して扇谷上杉家の領国支配の一翼を担い、相模国七沢要害（現神奈川県厚木市）に入った。