Born in 1539 as the eldest son of Chosokabe Kunichika, a provincial lord of Tosa. Mocked in youth as "the princess boy" for his gentle manner, he shocked everyone with ferocious valor at his first battle, the Battle of Nagahama in 1560. Succeeding his father, he developed the unique ichiryo-gusoku military system—farmers who fought in exchange for land rights—and advanced the unification of Tosa. By around 1575 he had fully controlled Tosa. He then expanded into Awa, Sanuki, and Iyo, and by 1585 had brought all four Shikoku provinces under his rule, becoming the only Sengoku lord to unify the island. That same year Toyotomi Hideyoshi launched an invasion with over 100,000 troops; Motochika surrendered within a month and was confined to Tosa alone. As a Toyotomi vassal he participated in the Korean campaigns. The death of his beloved heir Nobuchika at the Battle of Hetsugigawa in 1586 reportedly drained him of ambition. He sided with the Western forces at Sekigahara but did not engage in battle and escaped punishment. He died in 1599 at age 61.

Gentle in appearance but ferocious in battle. He deeply valued his retainers and their trust. Said to have lost his spirit after his beloved eldest son Nobuchika died in battle.

The only Sengoku lord to unify Shikoku. The ichiryo-gusoku system remains a notable military innovation. Though the clan fell after Sekigahara, the people of Tosa long revered them.