Because his mother was a cousin of the mother of Toyotomi Hideyoshi, he became a retainer of Hideyoshi, distinguishing himself in the attack on Tottori Castle in Inaba, in the battle against Akechi Mitsuhide at Yamazaki, and in particular at the battle of Shizugatake, where he won the distinction of being named among the shichihonyari, the seven most outstanding warriors of the battle. In 1586 he was made lord of Kumamoto Castle in Higo, and in the campaigns against Korea (1592 and 1597) he again won distinction, leading the vanguard in company with Konishi Yukinaga.
After the death of Hideyoshi, when a struggle for power developed between Ishida Mitsunari and Tokugawa Ieyasu, Kiyomasa and others of the militarist party proved useful to Ieyasu because of their opposition to Mitsunari And the civilian party, and although Kiyomasa did not take part in the battle of Sekigahara in 1600, he aided Ieyasu’s cause by putting down Mitsunari’s supporters in Kyushu; as a reward, he was confirmed by Ieyasu in his possession of the lands he held in Higo. Later he was ordered by Ieyasu to construct the keep of Nagoya Castle in Owari and served Ieyasu in other ways, but at the same time he continued to show concern for the safety of Hideyoshi’s heir, Hideyori.
For a time he also dispatched ships to carry on trade with Southeast Asia. His death in 1611 was a windfall for Ieyasu, clearing the way for the final overthrow of the Toyotomi family.
Though his talents were principally military, he was also an able administrator and won the admiration of the people of his domain by opening up the Kikuchi River for navigation and carrying out other projects that benefited them.