Yasutomi Mototsuna served as a bushō during the latter part of the Muromachi period. Mototsuna was a member of the Yasutomi clan serving as senior retainers of the Hosokawa-Keichō family (the main branch of the Hosokawa), inheriting the role of deputy military governor of the eastern portion of Sanuki Province in Shikoku and the landholdings. Mototsuna served as the kasai, or head of house affairs, and steward of the Hosokawa-Keichō family. He served Hosokawa Katsumoto and, as the head of house affairs, attended upon Katsumoto in the capital of Kyōto. In addition to Mototsugu, he may also have been the father of Yasutomi Motoie and his two younger brothers.
In 1467, after the outbreak of the Ōnin-Bunmei War, Mototsuna, along with the Kagawa clan, led forces from Sanuki and engaged in battle around Kyōto. The Yasutomi clan, along with the Noda clan of Nishioka, became servants of the Hosokawa-Keichō. Historical accounts include details of Mototsuna leading members of the Noda clan in military operations.
As the situation on the battlefield evolved in favor of the Western Army, on 10/3, these forces launched an all-out assault against the main base of the Eastern Army in an event known as Battle of Shōkoku Temple. Mototsuna led 3,000 mounted soldiers in defense of the Shōkoku Temple, but some of the monks colluded with the Western Army. A fire set by monks on the temple grounds shook the Eastern Army, leading to their collapse. Mototsuna fought valiantly against ferocious attacks by enemy soldiers, but, together with his younger brother, faced a valorous end. The Battle of Shōkoku Temple is regarded as one of the most violent clashes in the Ōnin-Bunmei War as detailed in chronicles of the battle.
Mototsuna was deeply trusted by Katsumoto so Katsumoto was greatly disappointed to learn of his death in battle. Other generals in the Eastern Army shared his lament at the loss.