His birth year is uncertain — either 1493 or 1501 — and he is said to have come from Mikawa or Suruga Province. One-eyed and lame, with an unprepossessing appearance, he nonetheless possessed outstanding talent in military strategy and castle construction. After wandering various provinces, he entered the service of Takeda Shingen (Harunobu) as a strategist, reportedly contributing greatly in fortification and tactics. At the Fourth Battle of Kawanakajima in 1561 he proposed the 'Woodpecker Strategy' (kitsutsuki senpo): a detached force would drive the enemy from their mountain position into the waiting arms of the main body below. But Uesugi Kenshin saw through the plan and the strategy failed. Taking responsibility, Kansuke charged into the enemy lines and was killed. He was around 60 years old. For centuries he was suspected of being a fictional character appearing only in the Koyo Gunkan chronicle, but in 1969 a letter by Takeda Shingen mentioning 'Yamamoto Kansuke' was discovered among the Ichikawa family documents in Hokkaido, confirming his historical existence.

Despite his unfortunate appearance, he was a devoted seeker of the martial arts, consumed by strategy and fortification. His tenacity — wandering for years until Shingen recognized him — and his warrior's pride in charging to his death to atone for a failed plan define his character.

Through the Koyo Gunkan he became legendary as one of the greatest strategists of the Sengoku era. The Woodpecker Strategy and his heroic death at Kawanakajima have been the subject of countless novels, dramas, and films — he was the protagonist of the 2007 NHK Taiga Drama 'Furin Kazan.' The discovery of the Ichikawa documents made him a symbol of the boundary between legend and historical fact.