Janssen was born in 1931 as David Harold Meyer in Naponee, a village in Franklin County in southern Nebraska, to Harold Edward Meyer, a banker (May 12, 1906 - November 4, 1990) and Berniece Graf (May 11, 1910 - November 26, 1995).
He attended Fairfax High School in Los Angeles, where he excelled on the basketball court setting a school scoring record that lasted over 20 years.