The third son of Kikkawa Motoharu, Hiroie became head of the Kikkawa house in 1587 and held a 140,000-koku domain across Izumo and Hoki. He began building Yonago Castle on Mt. Minato in 1591, but when Mori Terumoto was made nominal commander of the Western Army at Sekigahara in 1600, Hiroie secretly negotiated with the Tokugawa side and kept the main Mori force from moving at Mt. Nangu. This saved the Mori clan from destruction though drastically reduced, while Hiroie became lord of a 30,000-koku domain at Iwakuni. He never saw Yonago Castle completed. In 1608 he built Iwakuni Castle atop Mt. Yokoyama, only for it to be demolished seven years later under the One Castle Per Province decree.

A realist who prioritized the survival of the Mori clan above all, taking the considerable risk of independently negotiating with the Tokugawa. He was also an eager castle-builder, laying the foundations of both Yonago and Iwakuni castles.

His secret dealings at Sekigahara remained controversial within the Mori clan for generations, but ultimately saved the house from destruction. As lord of Iwakuni, he laid the foundations of the domain, which his descendants governed until the Meiji era.