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 wasted no time in marching on the Ichijō's headquarters at Nakamura, and Kanesada fled to Bungo, defeated. In 1575, at the Battle of Shimantogawa (Battle of Watarigawa), he defeated the Ichijo family. Thus he ended up gaining control of Tosa Province.

Following his conquest of Tosa, Motochika turned north and prepared for an invasion of Iyo province. The lord of that province was Kōno Michinao, a daimyo who had once been driven from his domain by the Utsunomiya clan, returning only with the assistance of the powerful Mōri clan. However, it was unlikely that Kōno could count on that sort of help again as the Mōri were embroiled in a war with Oda Nobunaga. Nonetheless, Chōsokabe's campaign in Iyo did not go off without a hitch.

In 1579, 7,000-man Chōsokabe army, commanded by Kumu Yorinobu, met the forces of Doi Kiyonaga at the Battle of Mimaomote. In the ensuing battle, Kumu was killed and his army defeated, though the loss proved little more than an unfortunate delay. The next year, Motochika led some 30,000 men into Iyo Province, and forced Kōno to flee to Bungo province.

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.

Later, Motochika advanced to Sanuki province defeated Sengoku Hidehisa at Battle of Hiketa. By 1583, Chōsokabe forces had subdued both Awa and Sanuki. Over the ensuing decade, he extended his power to all of Shikoku island, making Motochika's dream of ruling all of Shikoku a reality.