When Ieyasu entered the Kanto region, Tadatsugu followed him to become daikangashira (head of Edo-period prefectural governorship (magistrate, bailiff)), the predecessor of Kanto gundai (a magistrate of the Kanto region), playing a leading role in Tokugawa's rule of their Kanto territory.
家康の関東入国に従って代官頭（関東郡代の前身）となり、徳川氏の関東領国支配の中心的役割を担った。