As a minor noble, he originally served at the Imperial Court in Kyoto.

[2] On Hiromoto's advice, Yoritomo appointed jitō and shugo in 1185, which helped to strengthen shogunal control over the provinces.

In 1199 real power was moved from second shōgun Minamoto no Yoriie to the council of influential gokenin.

Hiromoto also helped the Hōjō clan crush enemies as Hatakeyama Shigetada, Hiraga Asamasa and Wada Yoshimori.

In the Jōkyū War he insisted on making a sudden attack to Kyoto and contributed to the shogunate's overwhelming victory.