Born in Sakata District, Omi Province (present-day Nagahama, Shiga), he was discovered by Toyotomi Hideyoshi as a boy and served as a page. The anecdote of the "three cups of tea" by which Hideyoshi discovered him is famous: he is said to have presented tea three times at progressively hotter temperatures, showing a wit that Hideyoshi admired. He thereafter served as Hideyoshi's trusted aide, demonstrating outstanding talent in administration, logistics, and diplomacy, and was granted the Sawayama domain in Omi worth 190,000 koku. During the invasions of Korea he crossed to the peninsula as commissioner of logistics, generating friction with the frontline commanders. After Hideyoshi's death, his conflict with Toyotomi generals such as Fukushima Masanori and Kato Kiyomasa deepened, and in 1600 he organized the western forces under Mori Terumoto as nominal commander to overthrow Tokugawa Ieyasu. At Sekigahara the western army initially held even ground, but the defection of Kobayakawa Hideaki caused total collapse. The fleeing Mitsunari hid in his native Omi but was captured and beheaded at Rokujo Kawara in Kyoto on 1 October of the same year, at age 40. The anecdote of his refusing a persimmon offered the night before his execution on the grounds that it was "bad for phlegm" is also famous, illustrating that he maintained a composed dignity to the very end.