Kato Kiyomasa was born in Owari Province to Kato Kiyotada. Kiyotada's wife, Ito, was a cousin of Toyotomi Hideyoshi's mother. Kiyotada died while his son (then known as Toranosuke) was still young. Soon after, Toranosuke entered service with Hideyoshi, and in 1576, at age 14, was granted a revenue of 170 koku. He fought in Hideyoshi's army at the Battle of Yamazaki, and later, at the Battle of Shizugatake. Owing to his distinguished conduct in that battle, he became known as one of the Seven Spears of Shizugatake. Hideyoshi rewarded Kiyomasa with an increased revenue of 3000 koku.
When Hideyoshi became the kampaku in the summer of 1585, Kiyomasa received the court title of Kazue no Kami (主計頭) and junior 5th court rank, lower grade (従五位下). In 1586, after Higo Province was confiscated from Sassa Narimasa, he was granted 250,000 koku of land in Higo, and given Kumamoto Castle as his provincial residence.
A devoted member of Nichiren Shu Buddhism, Kiyomasa encouraged the building of Nichiren temples. He did not see eye-to-eye with Ishida Mitsunari, and Hideyoshi recalled him to Kyoto. He came into conflict with Konishi, who ruled the neighboring domain in Higo, and was a Christian. Kiyomasa was noted for suppressing Christianity. At the battle of Hondo, he ordered his men to cut open the bellies of all pregnant Christian women and cut off their babies' heads.
During the Battle of Sekigahara, Kiyomasa remained in Kyuushuu, siding with the eastern army of Tokugawa Ieyasu. For his loyalty to the Tokugawa, Kiyomasa was rewarded with the former territories of his rival Konishi (who had sided with Ishida), which when added to his existing territory, increased the Kumamoto domain to around 530,000 koku.
In his later years, Kiyomasa tried to work as a mediator for the increasingly complicated relationship between Ieyasu and Toyotomi Hideyori. In 1611, en route by sea to Kumamoto after one such meeting, he fell ill, and died shortly after his arrival. He was buried at Honmyo-ji temple in Kumamoto, but also has graves in Yamagata Prefecture and Tokyo. Kiyomasa is also enshrined in a Shinto shrine in Kumamoto.