Subsequently, Tokimasa served Yoritomo as his important vassal, and after TAIRA no Kiyomori, who headed a Taira family of the Ise-Taira clan, was killed in 1185, he was ordered by Yoritomo to go up to Kyoto and negotiate with the Imperial Government for permission to set up a shugo (military governor) and jito (military land steward) in Japan.
その後は頼朝配下の有力重臣として活躍し、1185年（文治元）の伊勢平氏の平清盛一族（平家）の滅亡後、頼朝の命を受けて上洛し、守護と地頭の設置を認めさせるべく、朝廷との交渉に当たっている。