When Masamori succeeded as head of the family, the Taira family was not very strong, and served MINAMOTO no Yoshiie, under the Kawachi-Genji (branch of the Minamoto clan descended from the Seiwa-Genji).
正盛が家督を継いだ頃は平家も勢力が小さく、河内源氏に臣従し源義家に仕えた。