Archer, along with many other officers captured at Gettysburg, was eventually sent to the Johnson's Island prisoner of war camp on the coast of Lake Erie, where his health rapidly declined due to exposure to the inclement Ohio weather.
Archer, monien muiden Gettysburgissa vangittujen upseerien tapaan, lähetettiin Johnson's Islandin vankileiriin Eriejärven rannalle, missä hänen terveytensä romahti Ohion ilmastolle altistumisen vuoksi.