The younger brother of World War II correspondent and author Robert William St. John (1902–2003), Archer St. John was born either c. 1901 or on October 15, 1904 in Chicago, Illinois. Their mother Amy, a nurse, and father John, a pharmacist, moved the family to suburban Oak Park in 1910. Following the father's death in 1917 and the mother's eventual remarriage, Archer attended the St. Albans Episcopal Academy boarding school in Sycamore, Illinois. Both brothers became journalists, with Archer founding the "Berwyn" "Tribune" in the mid-1920s.