Kate was born in Bedford, New York. She is one of four children of Kathleen McNulty (Rooney) and NFL football team New York Giants executive Timothy Christopher Mara. Her younger sister is actress Rooney Mara.
Her grandfathers were Wellington Mara, co-owner of the Giants, and Timothy Rooney, owner of Yonkers Raceway, and her grand-uncle is Steelers Chairman Dan Rooney, the former US Ambassador to Ireland. She is the great-granddaughter of Art Rooney Sr., the founder of the Pittsburgh Steelers football franchise. She often sings the national anthem at Giants home games. Her father has Irish, German, and French-Canadian ancestry, and her mother is of Irish and Italian descent.
Mara graduated from high school a year early. She was accepted at the prestigious NYU Tisch School of the Arts but deferred her admission for three consecutive years.