After the death of his father Yoritomo in 1199, Sanetomo's grandfather Hojo Tokimasa usurped all political and military power of the shogunate, relegating the position and title of Seii Taishogun, or shogun, to a mere figurehead. Through hereditary succession, Sanetomo's older brother Yoriie became Seii Taishogun in 1202, to only be stripped of the title a year later and put under house arrest for plotting against the Hojo clan. Shortly thereafter in 1203, Sanetomo became head of the Minamoto clan and was appointed Seii Taishogun. In the next year, 1204, Yoriie was assassinated by the Hojo.
Sanetomo, understanding his own powerlessness in comparison to the Hojo and not wanting to meet the same fate as his brother, put all of his time and energy into writing poetry and gaining posts within the powerless but honorary imperial court. Sanetomo was a talented poet, writing over 700 poems and having one of his tanka included in the anthology 100 Poems by 100 Poets (Ogura Hyakunin Isshu), a collection of Japanese poems of the Heian and early Kamakura periods. Sanetomo also achieved the third highest post of the imperial court, Minister of the Right (Udaijin), a type of Vice-Minister, in 1218.
Sanetomo was assassinated on New Year's Day 1219 by his nephew, son of the second Kamakura shogun Yoriie, at the steps of Tsurugaoka Hachiman Shrine in Kamakura. With his assassination, the Seiwa Genji line of Minamoto came to an end.
Minamoto no Sanetomo was succeeded by Kujo Yoritsune as fourth shogun of the Kamakura shogunate.
Minamoto no Yoriie