After the death of Yoritomo in 1199, Sanetomo's maternal grandfather Hōjō Tokimasa usurped the political and military power of the Shogunate, relegating the position and title of Sei-i Taishōgun, or shōgun, to a figurehead.

Through hereditary succession, Sanetomo's older brother Yoriie became Sei-i Taishōgun in 1202, only to be stripped of the title a year later and put under house arrest for plotting against the Hōjō clan.

[citation needed] Under heavy snow on the evening of February 13, 1219 (Jōkyū 1, 27th day of the 1st month), Sanetomo was coming down from the Senior Shrine at Tsurugaoka Hachiman-gū after participating in a ceremony celebrating his nomination to Udaijin.

[2] For his act, he was himself beheaded a few hours later,[2] thus bringing the Seiwa Genji line of the Minamoto clan and their rule in Kamakura to a sudden end.

Minamoto no Sanetomo was succeeded by Kujō Yoritsune, son-in-law of Yoriie, as fourth shōgun of the Kamakura shogunate.