

Janssen starred in four television series of his own: He joined friend Martin Milner in a 1962 episode of Route 66 as the character Kamo in the episode "One Tiger to a Hill." Later, he guest-starred on NBC's medical drama The Eleventh Hour in the role of Hal Kincaid in the 1962 episode "Make Me a Place", with series co-stars Wendell Corey and Jack Ging.

In 1956, he and Peter Breck appeared in John Bromfield's syndicated series Sheriff of Cochise in the episode "The Turkey Farmers". Janssen appeared in many television series before he landed programs of his own. In TV series The Fugitive, 1963–1967 (final episode) During his Army days, Janssen became friends with fellow enlistees Martin Milner and Clint Eastwood while posted at Fort Ord, California. His first film part was at the age of thirteen, and by the age of twenty-five he had appeared in twenty films and served two years as an enlisted man in the United States Army. 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. Young David used his stepfather's name after he entered show business as a child. Following his parents' divorce in 1935, his mother moved with five-year-old David to Los Angeles, California, and later married Eugene Janssen (February 18, 1918 – March 30, 1996) in 1940 in Los Angeles. Janssen was born in 1931 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).
