Nishi was born as the son of a samurai of Satsuma Domain (present-day Kagoshima Prefecture), and as a youth was in the service of Shimazu Hisamitsu. He participated in the Battle of Toba–Fushimi during the Boshin War. Joining the fledgling Imperial Japanese Army in July 1871, he was made a battalion commander, and was promoted to major in December 1873. He subsequently participated in the suppression of the Saga Rebellion, the Taiwan Expedition of 1874, and the Satsuma Rebellion of 1877. Afterwards, he was assigned to the Imperial Japanese Army General Staff Office until February 1881. After a brief command of the IJA 11th Infantry Regiment, he was promoted to colonel in 1882 and to major general in 1889.