A native of Osumi District in Sagami Province (part of the present-day city of Hiratsuka, Kanagawa Prefecture), Yamanashi graduated from the former 8th class of the Imperial Japanese Army Academy in 1886.

After serving in a number of administrative positions within the Imperial Japanese Army General Staff, he was assigned as chief-of-staff of the IJA 18th Division at the Battle of Tsingtao in World War I.

Yamanashi also unsuccessfully attempted to challenge the entrenched concept within the Imperial Japanese Army that "spirit" (elan, or yamato-damashii) could prevail over deficiencies in modern weaponry or technology.

However, at the end of his tenure, the Imperial Japanese Army was still behind the other major powers in terms of mechanization and aviation.

[7] However, while in Korea, he became embroiled in a prominent corruption scandal in which he was accused of having accepted a very large bribe from a rice merchant who wanted to expand into the Korean market.