Shimada served with the Imperial forces in the Meiji Restoration war in 1868 as a soldier of the Kanazawa Clan and was later commissioned an army captain. When the government decided against an expedition to Korea, he returned to his native place in indignation. He unsuccessfully attempted to start a rebellion in response to the Saigo Rebellion in 1877.
Ichiro was famous as the lead assassin of the powerful politician Ōkubo Toshimichi on May 14, 1878. Following the assassination, he and the other assassins turned themselves in and all were executed in July of the same year.