She is a great-granddaughter of both New York Giants founder Tim Mara and Pittsburgh Steelers founder Art Rooney, Sr. Her mother's side of the family has held ownership in the NFL's Pittsburgh Steelers football team since its formation in 1933, and her father's side of the family has held ownership in the NFL's New York Giants since its formation in 1925. Her paternal grandparents were Wellington Mara and Ann Mara. Wellington co-owned the Giants football team from 1959 until his death in 2005, and was succeeded by his son (Kate Mara's uncle), John Mara, who is currently President, CEO, and co-owner of the team. Kate Mara's maternal grandfather, Timothy James "Tim" Rooney, has run Yonkers Raceway in Yonkers, New York since 1972. Her granduncle, Dan Rooney, chairman of the Steelers, was a former United States Ambassador to Ireland and the co-founder of the charitable organization The Ireland Funds. Her first cousin, once removed, Art Rooney II, is the current President and co-owner of the Steelers.