Aiō Mototsuna served as a retainer of the Mōri in Aki Province including, most notably, Mōri Motonari.  He was the third son of Mōri Hiromoto, a kokujin and landowner in Aki.  Mototsuna was not the lineal heir to Hiromoto, having been born to his mistress.  Mototsuna served as lord of Funayama Castle, adopting his name from the area he governed.  Mōri Okimoto and Mōri Motonari were Mototsuna’s older brothers born from Hiromoto’s formal wife. Known as a valiant warrior, Mototsuna joined his second oldest brother, Motonari, in the Battle of Arita-Nakaide in 1517, defeating Takeda Motoshige.

In the seventh month of 1523, Mōri Kōmatsumaru (the son of Okimoto), Mototsuna’s nephew and nominal head of the clan, died at the age of nine.  Motonari was then appointed as his successor. Senior retainers including Saka Hirohide and Watanabe Suguru maneuvered in support of Mototsuna.  Their plan was for Amago Toyohisa, a son-in-law of Mototsuna, to be appointed head of the clan, while Mototsuna would serve as his guardian. 

In the fourth month of 1524, Mototsuna and the others plotted to assassinate Motonari, but Motonari preempted the action by having Shiji Hiroyoshi lead a contingent of 300 men on a surprise attack of Mototsuna’s base at Funayama Castle.  Mototsuna, along with a small group of 30 retainers, furiously defended the castle, but ultimately Mototsuna was shot and stabbed with a spear.  According to an alternate theory, he died in a fit of anger based on a plot by the Amago between the eighth month of 1523 and third month of 1525.

Mototsuna’s son, Shikina Motonori, was pardoned owing to his young age, later becoming a retainer of the Mōri in Bingo Province.