Born in 1565 in Mino Province as the third son of Mori Yoshinari, a retainer of Oda Nobunaga. His real name was Naritoshi; 'Ranmaru' was his common name. In 1577, at age 13, he and his brothers entered Nobunaga's service as pages. Clever and perceptive, he won Nobunaga's deep trust. In 1582 he was granted Iwamura Castle in Mino at 50,000 koku—daimyo-level treatment for a youth. On June 2, 1582, the Honno-ji Incident erupted. When Akechi Mitsuhide's roughly 13,000 troops surrounded Honno-ji, Ranmaru fought alongside his lord to the end. After Nobunaga withdrew to the interior and took his own life, Ranmaru and his younger brothers Bomaru and Rikimaru continued fighting before being slain by Akechi retainer Yasuda Kunitsugu. He was 18.

Intelligent and quick-witted, with absolute loyalty to his lord. He won Nobunaga's trust through meticulous attentiveness while also demonstrating martial skill. An exceptional talent who received daimyo-level holdings at a young age.

Remembered as a symbol of loyalty for falling with Nobunaga at Honno-ji. His death at just 18 is etched in the Japanese imagination as a tale of youth, devotion, and the transience of the Sengoku era.