When in 1581 his father died, Hideie became the head of the Ukita household, and when he was 11 fought his first battle alongside Hashiba Hideyoshi at Takamatsu castle in Bitchu, at the head of 10000 men.

Ukita would fight in the Kyushu and Odawara campagins, and would become the commanding officer during the Korean campaign after Hideyoshi decided against going himself. He did well in Korea, and from his base in Seoul, made several important decisions, such as divisions of labor, sending out some of his headquarter generals to retake Sonsan, and the evacuation of Seoul. He also won a major battle at Pyokje, though it was followed by a major loss at Haengju.

Doted on and raised by Hideyoshi, he was given the Toyotomi surname and the "Hide" from Hideyoshi's name as a sign of affection and trust. He was further named as one of Hideyoshi's Tairo. It was thus only natural that Hideie would be on the loyalist army's side during the battle of Sekigahara.

With 17000 men under him, Hideie was at the head of the largest single unit in the western army. Situated strategically on Mount Tengu, Hideie's troops battled with Fukushima Masanori's men at the scene of some the most ferocious fighting, until attacked on his flank by the turncoats, led by Kobayakawa Hideaki. Ukita was outraged at this betrayal, and wanted to fight one-on-one with Kobayakawa, and had to be pulled away by his advisors. He was praised by Mitsunari for his conduct during the battle: "Ukita .. and I held our ground and fought on without confusion to the very last, and never let our defeat become a rout. So whatever abuse is levelled at us we have nothing to be ashamed of."