Born on January 22, 1553, as the eldest son of Mori Takamoto. His grandfather was Mori Motonari, hegemon of western Honshu. When his father died suddenly in 1563, 11-year-old Terumoto became head of the Mori clan, raised under the guidance of Motonari and his uncles Kikkawa Motoharu and Kobayakawa Takakage. He submitted to Toyotomi Hideyoshi, became one of the Five Elders (Gotairo), and held 1,205,000 koku. At the Battle of Sekigahara in 1600, he was installed as nominal commander-in-chief of the Western Army and entered Osaka Castle, but he never took the field; Kikkawa Hiroie's secret dealings with the Tokugawa ensured the Mori forces stayed immobile. After the defeat, his domain was slashed to Suo and Nagato provinces at 298,000 koku. He relocated to Hagi and began building Hagi Castle in 1604. He died on April 27, 1625, aged 73. The Mori house survived as the Hagi (Choshu) domain until the end of the Edo period and became a driving force of the Meiji Restoration.

Cautious and conservative, adhering to his grandfather Motonari's precepts. He tended to be swayed by those around him rather than acting decisively, but his overriding priority of preserving the Mori house ultimately proved correct.

Known as the 'general who didn't fight' despite being the Western Army's commander-in-chief at Sekigahara. Yet by preserving the Mori house through severe domain reduction, he ensured that the Choshu domain would later become the spearhead of the Meiji Restoration—an immeasurable legacy.