Siedow was born in Cheyenne, Wyoming. He began acting in his high school drama class, and later on, moved to New York City, New York, where he started appearing in theatre productions. During World War II, Siedow served with the Army Air Corps, and after the war, Siedow moved to Chicago, Illinois and started doing radio work. In September 1946, he met and married an Actress named Ruth, his wife up to his death. They had three children.