Reiner was born Tracy Henry in Albuquerque, New Mexico, the daughter of Penny Marshall and Michael Henry. She is filmmaker Garry Marshall's niece. Her mother was married for 10 years to Rob Reiner, who adopted her and raised her as his own. Her mother is of Italian, English, German and Scottish descent. Reiner's adoptive paternal grandparents are Carl Reiner (1922–2020) and Estelle Reiner (1914–2008). Growing up, 15 of Reiner's relatives were employed at labs run by the United States Atomic Energy Commission.