Yasutomo served Kamakura bakufu and was promoted to hisashi ban (a person who keeps the night watch staying under the eaves of building), a member of banyaku that works as close advisors of Shogun on December 24, 1257, and June 17, the next year, he was ordered to accompany Shogun to Tsurugaoka Hachimangu Shrine where Tsurugaoka-Hojoe (the ritual for releasing living beings) was held.
鎌倉幕府に仕え、正嘉元年（1257年）12月24日に廂番（ひさしばん）という将軍の側近に使える番役衆に任命され、翌年の6月17日には、鶴岡放生会参詣の供奉人に命ぜられている。