By the late 60s, he had settled in NYC and began to pursue a stage acting career, appearing in the Broadway company of "Hair" and off-Broadway in David Rabe's "Sticks and Bones" (1971), a role he reprised in the 1973 CBS broadcast of the play. Many affiliates found the subject matter (about a returning Vietnam veteran) too controversial and refused to air the production. During his days in NYC, De Young also landed a regular role on the CBS daytime drama "The Secret Storm".
By 1973, De Young had relocated to California where he quickly established himself in TV. In 1973, he starred as a struggling musician forced to care for the daughter he fathered in "Sunshine" (NBC), which was turned into a short-lived 1975 NBC series. In 1977, he returned to the role for "Sunshine Christmas". A frequent player in TV-movies and miniseries, his credits include portraying historical figures like Charles Lindbergh in "The Lindbergh Kidnapping Case" (NBC, 1976), Robert F. Kennedy in "King" (NBC, 1978) and John F Kennedy in "Robert Kennedy and His Times" (CBS, 1985). Among his more notable fictional creations are John Skimmerhorn in the NBC miniseries "Centennial" (1978-79), Dyan Cannon's loyal assistant in "Master of the Game" (CBS, 1984) and Joe in "Stephen King's 'The Tommyknockers'" (ABC, 1993). In 1996, De Young was a sergeant captured by the Confederates and placed in the prison camp "Andersonville" (TNT, 1996) and homicide detective convinced Ann-Margret is a murderer in "Seduced By Madness: The Diane Borchardt Story" (NBC, 1996). Later that year, he returned to series TV as the somewhat distant father of a young woman in love in the ABC drama series "Relativity". He signed with personal manager Michael Wallach that same year and in 1997 starred in the miniseries "Wallace" (TNT) and "The Last Don" (CBS).
On the big screen, De Young has alternated between studio films and low-budget productions. He made his debut in the independent "Pilgrimage" (1972) and went on to featured roles in "Harry and Tonto" (1974) and "Blue Collar" (1978). De Young has appeared in several genre films, like "Shock Treatment" (1981), the sequel to "The Rocky Horror Picture Show", in which he replaced Barry Bostwick as Brad, and "Pulse" (1988), as Joey Lawrence's disbelieving father. In 1989, he was in the prestige film "Glory" as a Union colonel who uses his 'Negro' troops to loot and pillage. De Young was the missing husband of Mimi Rogers in "Fourth Story" (1991) and co-starred in "The Substitute" (1996), as a conspirator in the drug pushing that Tom Berrenger seeks to eradicate.