Hideaki was the 5th son of Toyotomi Hideyoshi's brother-in-law Kinoshita Iesada (1543-1603). He was mentored by Kuroda Yoshitaka and adopted by Kobayakawa Takakage, who would bequeath to him a 336,000-koku fief on Kyushu upon his death (1596). Hideyoshi named Hideaki nominal commander of the Second Korean Campaign (1597-98), with Kuroda acting as his advisor. While serving in this capacity, Hideaki was harshly criticized by Inspector of Forces Ishida Mitsunari in his dispatches to Hideyoshi, earning Hideaki's deep animosity. There may have been something to Mitsunari's reports, for Hideyoshi arranged for Hideaki to be relocated from Kyushu to Kita no sho in Echizen, with a consequential drop in income from 336,000 koku to 120,000 koku. Just prior to his death, Hideyoshi decided to retract his earlier decision and allowed Hideaki to return to Kyushu and assume control of his original domain, which included lands in Chikuzen, Chikugo, and Buzen. Tokugawa Ieyasu is said to have intervened on Hideaki's behalf on at least one occasion during the latter's period of disgrace.
When sides were drawn in 1600 between Tokugawa Ieyasu and Ishida Mitsunari, Hideaki wavered. Ishida, mindful no doubt of their troubled history, offered Hideaki the title of Kanpaku and guardianship of the late Hideyoshi's son Hideyori - impressive offers indeed. Interestingly, when Hideaki approached Hideyoshi's widow for advice, she hinted that he would do best to side with the Tokugawa. He ultimately decided to join Ieyasu but was intercepted by Mitsunari himself and convinced to give his support to the 'western' cause. That support could prove decisive, for Hideaki had nearly 16,000 men under his command-or a force comparable to many earlier sengoku armies. He satisfied Mitsunari with his sincerity by lending his strength to the reduction of Fushimi.
Hideaki's agreement, however, was only a ruse - he sent a letter to Ieyasu promising that in the coming battle, he could be counted on to betray his erstwhile allies. At Sekigahara (21 October, 1600) Hideaki took up position on Mt. Matsuo with his command of 15,600 while below waited some 4,000 more under commanders also determined to switch sides. In the event, Hideaki hesitated while ignoring pleas from Mitsunari to attack. The fighting began at around eight in the morning, and was not until noon that Hideaki committed his forces to battle-and only then after prodding by Tokugawa arquebusiers. Nonetheless, his betrayal was decisive - he overran the small command of Ôtani Yoshitsugu and attacked Ukita Hideie's hard fighting troops in the flank. Ukita broke, and by 1400 the battle was won.
Afterwards, Tokugawa received Hideaki cordially and then requested his command pursue the fleeing 'western' survivors. Hideaki took his army to Sawayama, which fell after the suicide of Ishida Masazumi.
Ieyasu clearly kept Hideaki at an arm's length following the campaign, and it is interesting to speculate on the former's true feelings. Hideaki had after all contributed to the fall of Fushimi and the death of the veteran Tokugawa retainer Torii Mototada. Certainly, relative to the importance of his defection, Hideaki's reward was slim: an additional 50,000 koku, but then, few of the turncoats received much in the way of a reward - aside from retaining fiefs that would have otherwise been lost. At any rate, Hideaki was not particularly popular following the battle, and is supposed to have gone mad prior to his death in 1602.
Like Ukita Hideie, Hideaki was groomed for power by Hideyoshi, who referred to him in his letters as the Tamba Chûnagon. He is a little explored figure and surrounded by questions even today.