The eldest son of Ōtomo Yoshiaki [ja], he inherited the Funai Domain, on Kyūshū, Japan's southernmost main island, from his father.

[1] Sōrin's domain included the port of Funai, which was frequented by Jesuit priests, bandits, Chinese merchants, and Japanese sea lords.

Though he later formally converted to Christianity, it is likely that Sōrin saw this as a strategic move, politically, and was not religiously motivated towards this position[citation needed].

Referred to as the "King of Bungo" in the Jesuit records, Sōrin sent political delegations to Goa in the 1550s, and the Tenshō embassy to Rome in 1582.

In 1557, Ōuchi Yoshinaga (Sorin's younger brother) was forced to commit suicide by Mōri Motonari, and in 1558 Mori captured Moji castle from the Ouchi.

By this time, Sōrin controlled Bungo, most of Buzen, Chikuzen, and Chikugo, and had influence over Hugo, Hizen and Iyo, soon became known as the "Seven-Province Host of the Ōtomo".

[7] Soon after his conversion, Sōrin and a force of samurais traveled to Tsuchimochi in Hyūga with the aim of establishing a new society based on European Christian principles.

In 1587, Hideyoshi began his Kyūshū Campaign, in which he overtook the entire island, with the help of the Ōtomo and other families which voluntarily entered his service.