Born in 1545 as the son of Yamanaka Mitsuyuki, a retainer of the Amago clan. Known in childhood as Jinjiro, he showed martial talent from a young age. When the Amago stronghold, Gassan-Toda Castle, fell to Mori Motonari's forces in 1566 and the clan was destroyed, Yukimori became a masterless samurai and dedicated himself to restoring the Amago house, rallying around Amago Katsuhisa — a surviving clan member then living at Tofukuji temple in Kyoto — and leading three separate restoration campaigns over the following years. His legendary prayer to the crescent moon — "Grant me, I beg you, seven calamities and eight hardships" — made him famous in later generations as a paragon of loyalty. In 1576 he pledged allegiance to Oda Nobunaga and campaigned under Akechi Mitsuhide and Hashiba Hideyoshi. In February 1578, when Hideyoshi captured Kozuki Castle in Harima, Yukimori was entrusted with its defense alongside his lord Amago Katsuhisa. That April, however, a Mori army of over 30,000 surrounded the castle, and Hideyoshi's relief force withdrew to prioritize the siege of Miki, leaving the defenders isolated. The castle surrendered on July 5; Katsuhisa was made to commit seppuku, and Yukimori, taken hostage for transport to Mori Terumoto, was murdered en route by the Mori retainer Fukuma Motoaki at the Ai (Ko) river crossing in Bitchu Province (present-day Takahashi, Okayama Prefecture). He was 34 years old.