He was the 21st chief of the Chōsokabe clan of Tosa Province (present-day Kōchi Prefecture), the ruler of Shikoku region.

In the course of the decade he was awarded the court rank Ministry of Imperial Household (Kunai shō) and was sufficiently confident after the reduction of Aki to finally turn on the Ichijō family.

In 1573, while still lord of the Hata district of Tosa, Ichijō Kanesada was unpopular and had already suffered the defection of a number of important retainers.

Seizing the opportunity, Motochika with his brother Kōsokabe Chikayasu, wasted no time in marching to attack on the Ichijō's headquarters at Nakamura, and Kanesada fled to Bungo, defeated.

In 1579, a 7,000 men Chōsokabe army, commanded by Kumu Yorinobu, met the forces of Kōno Michinao, led by Doi Kiyonaga at the Battle of Mimaomote.

With little interference from either the Mōri or the Ōtomo, Chōsokabe was free to press onwards, and in 1582, he stepped up ongoing raids into Awa province and defeated Sogō Masayasu and the Miyoshi clan at the Battle of Nakatomigawa.

In 1596, the Spanish ship San Felipe was wrecked in Chōsokabe territory while en route from Manila to Acapulco.

In addition to his leadership, Motochika is remembered for his '100-Article Code of the Chōsokabe' and his struggle to found an economically strong castle town, moving in the course of his career from Oko to Otazaka and on to Urado.

[6] Motochika is also a playable character in the Sony PlayStation game, Sengoku Basara where he wields an anchor and appears as a pirate.