In 1936, Koch began working as a guard and secretary at the Sachsenhausen concentration camp near Berlin, which her fiance, Karl-Otto, commanded. They got married the same year. In 1937, Koch went to Buchenwald when Karl-Otto was made Commandant there. While at Buchenwald, Koch helped with an experiment by picking out tattooed prisoners to be murdered and skinned, so the tattooed parts of their skin could be kept. Koch later said she did this to help a prison doctor, Erich Wagner, in his study of tattooing and crime.