He was born at Kōriyama Castle in Yoshida, the second son of Mōri Motonari, a local lord of Aki. His mother was Myōkyū, daughter of Kikkawa Kunitsune, and his full brothers were the elder Mōri Takamoto and the younger Kobayakawa Takakage. The Mōri were then merely one lord in Aki, struggling to survive between the great powers of the Ōuchi in the west and the Amago in the east.
Amago Haruhisa of Izumo attacked Kōriyama Castle with a large army. Motoharu, still a boy who had not yet come of age, is said to have gone into battle over his father's objections and fought splendidly in his first engagement. With Ōuchi reinforcements, the Mōri drove off the Amago, and this battle raised their standing in Aki.
He came of age with his brother Takamoto placing the cap on his head, and taking the character "Moto" of the Mōri line he called himself Mōri Motoharu. At this time Kikkawa Okitsune, head of his mother's family, had broken with the Ōuchi, and his father Motonari was concerned about how to deal with the Kikkawa house.
With the Kikkawa retainers split into two factions, his father Motonari's maneuvering led to an agreement that Motoharu would be adopted by Kikkawa Okitsune and inherit the house. That same year he married the daughter of Kumagai Nobunao, a warrior famed for his courage. An anecdote says that although she was rumored to be plain, Motoharu himself wished to marry her.
Motoharu entered Hiyama Castle, the Kikkawa base, and formally became head of the Kikkawa house. That year Okitsune, who had been made to retire, lost his life together with his family at the hands of men acting on Motonari's orders. Thus the system of the "two rivers" took shape, with the Kikkawa and the Kobayakawa, inherited by his brother Takakage, supporting the Mōri.
With his father Motonari and others he met the large army of Sue Harukata, who had overthrown his lord Ōuchi Yoshitaka, at Itsukushima in Aki. Leading the Kikkawa forces in concert with the Kobayakawa, Motoharu helped defeat Sue's army in a surprise attack. Victory at the Battle of Itsukushima propelled the Mōri to become a great power in the Chūgoku region.
While the Mōri invaded Suō and Nagato, he was sent to Iwami to guard their rear. He held off the army of Amago Haruhisa, which attacked aiming at the Iwami silver mine, and though once defeated, he rushed to relieve Yamabuki Castle and drove the Amago back. Thus Motoharu came to bear responsibility for the San'in front.
In camp during the war with the Amago, he began copying by hand all forty volumes of the Taiheiki, the war tale of the Nanboku-chō conflicts, completing it over two years. This manuscript, known as the Kikkawa text, survives today and, preserving an old form of the work, is designated an Important Cultural Property.
Fighting as a main force in the Battle of Gassan-Toda Castle, he finally forced its lord Amago Yoshihisa to surrender. The Amago, who had threatened the Mōri since his grandfather's day, thus lost their power as a daimyō house. Entrusted with the politics and military affairs of San'in provinces such as Izumo and Hōki, Motoharu became the Mōri commander in San'in.
An army led by Yamanaka Yukimori and others, seeking to restore the Amago house, rose in San'in, and Motoharu was kept busy suppressing it. That same year Ōuchi Teruhiro, who had been sheltered by Ōtomo Sōrin in Kyushu, invaded Suō, but Motoharu struck swiftly before Ōtomo reinforcements could gather and drove Teruhiro to his end.
His father Motonari died, and his nephew Mōri Terumoto, son of his late elder brother Takamoto, became head of the house. Motoharu, with his brother Takakage, took on the role of supporting the young Terumoto. Terumoto deeply respected his two uncles, and they exchanged written oaths to strengthen their unity.
Hashiba Hideyoshi, acting on Oda Nobunaga's orders, invaded the Chūgoku region, and Amago Katsuhisa and Yamanaka Yukimori entered Kōzuki Castle on the Oda side. Motoharu and others besieged and took the castle; Katsuhisa took his own life, and Yukimori, his longtime foe, also met his end. Thus the movement to restore the Amago came to a close.
He marched to relieve Bitchū Takamatsu Castle, which Hideyoshi had flooded, but a standoff continued. After the Honnō-ji Incident, the Mōri made peace with Hideyoshi. Afterward Motoharu handed headship to his eldest son Motonaga and retired, reportedly out of reluctance to serve Hideyoshi, though the timing of his retirement is debated.
Though retired, he answered the call to join Hideyoshi's Kyushu campaign and took Kokura Castle in Buzen. But he fell gravely ill in camp and ended his life at Kokura Castle. Hideyoshi sent a courteous letter of condolence, praising his death in camp during the cold season as an act of loyalty.
Mōri MotonariŌuchi YoshitakaOda NobunagaMōri TerumotoŌtomo SōrinKobayakawa Takakage
Akechi MitsuhideRyuzoji TakanobuUesugi KenshinŌtomo SōrinLuís FróisKobayakawa TakakageAsakura YoshikageShimazu Yoshihisa
People of the Sengoku/Azuchi-Momoyama period ・ People by period
Based on facts found in textbooks and biographies. Uncertain points are marked as such.