[4]: 213, 220–221 In 1566, he claimed Izumo Province as his fief, after defeating its lords, the Amago clan, in a number of battles.
Strong and fearless, Motoharu participated in many battles such as Miyajima and Nunobeyama, bolstering his reputation as a valiant commander.
According to the Meisho Genko Roku, his brawn was often contrasted with Kobayakawa Takakage's intellect back in their youth.
This inspired Mori Motonari to make effective use of their differences in personality by relegating Motoharu to the front lines while leaving Takakage to provide strategic support from the rear.
Although Motoharu heavily contributed to the Mōri's military growth, he was familiar with literature and even transcribed a series of books called the Taiheki during the siege of Gassantoda Castle.