Born in 1577 in Owari as the son of Mori Katsushin. He served Toyotomi Hideyoshi, and with his father held Kokura in Kyushu at 60,000 koku. After siding with the Western Army at Sekigahara and losing his domain, he was placed in the custody of the Yamauchi of Tosa. In 1614, leaving his wife and children as hostages, he escaped Tosa and entered Osaka Castle, becoming one of the Five Generals of Osaka. On May 7, 1615, at the decisive Battle of Tennoji, he led 4,000 troops and routed Honda Tadatomo and Ogasawara Hidemasa in quick succession, driving all the way to Ieyasu's headquarters. His breakthrough is credited with enabling Sanada Yukimura's famous charge at Ieyasu's camp. After Osaka Castle fell, he performed the kaishakunin role for his lord Hideyori and then took his own life. He was 39.

Calm, composed, and a gifted field commander. Though overshadowed by Sanada Yukimura in fame, his battlefield results are said to have equaled or surpassed Yukimura's. Loyal to the end, he upheld his duty to the Toyotomi house.

As the saying goes, 'Posterity speaks of Sanada but not of Mori'—he is often overshadowed by Yukimura, yet his exploits at Tennoji are considered the greatest feat of arms in the Summer Campaign. Even contemporary missionary accounts recorded his valor.