In February 1199, when Yoritomo died and MINAMOTO no Yoriie became the second Seii Taishogun (literally, "great general who subdues the barbarians"), Yoshimori became a member of the 13-person parliamentary system as Shukuro (chief vassal).
建久10年（1199年）正月に頼朝が死去し、源頼家が二代征夷大将軍になると義盛は宿老として十三人の合議制に列した。