A native of Kagoshima prefecture, Samejima was the younger son of a samurai of Satsuma Domain. He joined the fledgling Imperial Japanese Army in 1871, and rose up from the ranks, starting with corporal in June 1873. He attended the Imperial Japanese Army Academy in March 1874 and was commissioned as an ensign with the Tokyo Garrison. He was a participant of the Japanese invasion of Taiwan (1874) and promoted to second lieutenant. He fought against his fellow Satsuma clansmen in the suppression of the Satsuma Rebellion in 1877 and was promoted to lieutenant in military engineering in the IJA 1st Infantry Brigade. In March 1879, he was assigned to the Imperial Japanese Army General Staff and promoted to captain in April 1881. He subsequently served in numerous staff positions, including on the Imperial Guard and as vice-commandant of the Army Staff College. He served in combat during the First Sino-Japanese War of 1894-1895 and was promoted to major general in September 1897.