Born as Shojumaru, in a small domain of Aki province. His father, Mōri Hiromoto, died when he was 9, therefore leaving the clan in Motonari's older borther, Mōri Okimoto. Okimoto held an allince with Ouchi Yoshioki (a powerful daimyō to the west of the Mori lands) founded by Hiromoto and assited Ouchi Yoshioki in Kyoto. In 1516 Okimoto died and Motonari became the guardian of Okimoto's son, Komatsumaru. After Komatsumaru died, Motonari became the head of Mōri clan.
Sandwiched between powereful Amago clan and Ouchi clan, Motonari led the clan by careful balancing acts and diplomacy. Eventually, Motonari succeeded in defeating both and controlled entire Chugoku area. In his late years, he crushed against Otomo clan of Fungo province.
He was gifted with three sons each with their own strong points. He preached three sons to work together for the mori clan. In one of the episode, he handed each of his sons an arrow and asked for them to snap it. After each snapped their arrows, he showed three arrows and asked them to snap them. Unable to snap them, Motonari preached how one arrow could be broken easily but three arrows could not.
His eldest son, Mōri Takamoto died of sudden disease, some say poisoning, while on their route to attack Amago clan. Saddened and angered by his death, Motonari ordered all those responsible to be punished.
Mori Hiromoto, the lord of Aki, has sworn his allegiance to Ouchi Yoshioki, but Mori is aware that Ouchi is a devious man. Unable to find comfort in the arms of his wife, Lady Sachi, who is seriously ill, Mori takes to drinking and visiting with his concubines.
Motonari journeys with his sons to Yamaguchi to inform the Ouchi of Motoharu's ascension as the lord of the Kikkawa Clan and Takamoto's marriage to Toshi.
Motonari slaps his son across the cheeks declaring that he deserves to be executed for such a rude display of anger.