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.
Shigeo Samejima participated in the Imperial campaign against the Tokugawa Shogunate at the Meiji Restoration (1868).
He was a participant of the 1874 Taiwan Expedition and promoted to second lieutenant. He fought 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.
In the Russo-Japanese War (1904-1905), he commanded the Eleventh Division which took part in the siege of Port Arthur.
In April 1906, he was awarded the Grand Cordon of the Order of the Rising Sun and in June 1906 was transferred to command the IJA 14th Division. In September 1907, Samejima was elevated to the kazoku peerage with the title of baron (danshaku).
In September 1911, he was promoted to full general, and entered the reserves the same year.
He officially retired in April 1916.
Samejima had no children.