Erik-Ørn Gjems-Onstad was born in Kristiania (modern-day Oslo) to jurist and civil servant Olaf Gjems-Onstad (1882-1945) and architect Ågot Urbye (1886-1959).
Gjems-Onstad was active in the boy scouts before the invasion, and he was because of his experience there asked to instruct in map and compass for a volunteer military training program in late 1939.