His father, Sol Horowitz, and paternal grandparents were Russian Jews. Horowitz's American-born mother, Ethel (née Frankel), and his maternal side of the family were Romanian Jews. Some of his relatives died during the Holocaust. He is the husband of author Cynthia Palmer, and the father of Winona Ryder and Uri Horowitz.
A former close associate of Timothy Leary, he is responsible with his wife for the creation of the world's largest library of drug literature, the Fitz Hugh Ludlow Memorial Library. He currently is the owner of Flashback Books, a mail-order bookselling operation, specializing in rare, scarce and out-of-print books, periodicals and related printed materials on the history, literature and science of psychoactive drugs.