Yasutomi Morisada served as a bushō during the Sengoku and Azuchi-Momoyama periods He was the lord of Amataki Castle in Sanuki Province. Morisada was born as the son of Yasutomi Morikata.
The Yasutomi clan were senior retainers of the Hosokawa-Keichō family. Around the time of Yasutomi Morinaga, the family inherited the role of deputy military governor of eastern Sanuki Province. Morinaga was revered as one of the Four Heavenly Kings of the Hosokawa – four retainers of Hosokawa Katsumoto who served valorously during the Ōnin-Bunmei War (with the others being Kōzai Motosuke, Nara Motoyasu, and Kagawa Motoaki). However, the landholdings of the Kōzai clan had, from the outset, exceeded those of the Yasutomi. Moreover, in eastern Sanuki, the Sangawa, the Ueda, and other clans were holding their ground against one another so the authority of the Yasutomi as deputy military governors rapidly waned, while a portion of the territory of the Yasutomi had been allocated from the Sangawa clan, leading to ongoing conflict. Finally, the Sogō clan (a branch of the Ueda), became relatives of the Miyoshi clan who were the deputy military governors of neighboring Awa Province. During the Tenbun era (1532 to 1555), the Sogō acquired the power to overwhelm other clans in eastern Sanuki so the Sangawa, the Kōzai, and the Yasutomi clans all submitted to the Sogō clan. In 1544, Morisada became the head of the family after the death of his father, Morikata.
In the third month of 1562, Morisada participated on behalf of the Miyoshi clan in the Battle of Kumeda, followed, in the fifth month, by the Battle of Kyōkō Temple.
In 1570, Morisada received as his formal wife the daughter of Shinohara Nagafusa (a senior retainer of Miyoshi Nagaharu). In 1571, upon orders of Nagaharu, four neighborhoods in the Ōchi District governed by Sangawa Motochika were transferred to the Miyoshi family, after which Yano Kunimura was assigned to Hiketa Castle and Morisada was assigned to Toramaru Castle. These transfers were conducted for the purposes of reinforcing the control through Shinohara Nagafusa of the Awa-Miyoshi clan in Sanuki and securing the route for a deployment to Bizen Province for the Siege of Motobuto Castle later that year. Morisada obeyed orders by moving to Toramaru Castle while a retainer of the Yasutomi named Rokusha Sōtan entered Amataki Castle.
In 1573, however, Shinohara Nagafusa was killed by Miyoshi Nagaharu at the Siege of Uezakura Castle. Thereafter, Nagaharu experienced a sudden loss of his power. In the third month of 1577, Hosokawa Saneyuki (Nagaharu’s half-brother of a different father), backed by Chōsokabe Motochika, defeated Nagaharu who then killed himself. Nagaharu’s younger brother, Sogō Masayasu, subsequently entered Shōzui Castle and controlled commanders in Awa and Sanuki but, in 1582, lost Awa after a defeat at the Battle of Nakatomigawa.
In 1583, in a bid to assist Masayasu, Sengoku Hidehisa (a retainer of the Oda clan) went to Shōdo Island which was controlled by Morisada, but was defeated by Chōsokabe forces led by Kagawa Nobukage and Kōzai Yoshikiyo at the Battle of Hiketa. At this time, Amataki and Toramaru castles fell to the Chōsokabe army. Later, he was killed in action while fighting on behalf of Sengoku Hidehisa against the Shimazu army at the Battle of Hetsugigawa in northern Kyūshū.