Born in Brooklyn, New York, to voice actor Allen Swift, Stadlen studied acting with Sanford Meisner and Stella Adler.
Other noted Broadway roles include Senex in A Funny Thing Happened on the Way to the Forum, Banjo in a revival of The Man Who Came to Dinner, Milt in Laughter on the 23rd Floor, and Dr. Pangloss in the 1973 production of Candide.
The Time of Your Life was revived on March 17, 1972, at the Huntington Hartford Theater in Los Angeles where Stadlen, Henry Fonda, Richard Dreyfuss, Ron Thompson, Strother Martin, Gloria Grahame, Jane Alexander, Richard X. Slattery and Pepper Martin were among the cast with Edwin Sherin directing.
Stadlen's film credits include Portnoy's Complaint (1972), Serpico (1973), The Verdict (1982), To Be or Not to Be (1983), Windy City (1984), and In & Out (1997).
On television, Stadlen had a regular role in the first season of Benson as John Taylor, Governor Gatling's chief of staff, before being replaced in the second season by René Auberjonois, who assumed the role of Clayton Endicott, Taylor's replacement as chief of staff, on the series.