Leoni was born on February 25, 1966 in New York City. Her mother, Emily Ann (née Patterson), was a dietitian and nutritionist, and her father, Anthony Pantaleoni, was a corporate lawyer with the firm Fulbright & Jaworski. Her paternal grandfather was of Italian, English and Irish descent; he was a nephew of Italian economist and politician Maffeo Pantaleoni. Leoni's paternal grandmother, Polish-American Helenka Adamowska Pantaleoni, a film and stage actress, was the daughter of musicians Józef Adamowski and Antonina Szumowska-Adamowska. Leoni's mother is a native of Amarillo, Texas, and a niece of actor Hank Patterson.