Born to German Mennonite parents in Friedensdorf (now Khmelnytskyi), Ukraine, Reimer studied to be a librarian before being drafted into the Soviet Army in 1940. When Germany invaded the Soviet Union on June 22, 1941, Reimer entered combat and was captured by German forces on July 6. Two months later, due to his ethnic German heritage and language skills, he was recruited to the Trawniki concentration camp for training as a camp guard.