Former brother-in-law of Donald Mark Spencer.
He has the distinction of being the only one of the two boys who appeared on Irwin Allen's TV series to be doubled. He and Bill Mumy were the only boys to appear in Allen's series, and Mumy had the distinction of being the only one of the two boys to appear as his past self (in the "Lost in Space" (1965) episode "Project: Earth"), in a flashback to the time of the Jupiter Two's original launch. But Stefan had to be doubled for in the "Land of the Giants" (1968) episode "Wild Journey" because he had already gone into puberty and thus could not appear as his younger self.
He is an accomplished songwriter. He wrote several songs with the late Warren Zevon on the album "Transverse City". And worked with many accomplished musicians.
Older brother of Alison Arngrim.
Son of Norma MacMillan and Thor Arngrim