Weitz was born in New York City, the son of actress Susan Kohner and Berlin-born novelist/menswear designer John Weitz. His brother is filmmaker Paul Weitz. Weitz is the grandson of Czech-born agent and producer Paul Kohner and actress Lupita Tovar on his maternal side. Tovar, who was from Oaxaca, Mexico, starred in "Santa", Mexico's first talkie, in 1932, as well as a Mexican version of "Drácula." Weitz' paternal grandparents escaped Nazi Germany, before which his grandfather was a successful textile manufacturer, with the family being intimates of writer Christopher Isherwood and actress Marlene Dietrich.