Maddow was born in Castro Valley, California. Her father, Robert B. Maddow, is a former United States Air Force captain who resigned his commission the year before her birth and then worked as a lawyer for the East Bay Municipal Utility District. Her mother, Elaine (née Gosse), was a school program administrator. She has one older brother, David. Her paternal grandfather was from a family of Eastern European Jews (the original family surname being "Medwedof") who self-identified as Polish and Russian at the time of their arrival in the United States. Her paternal grandmother was of Dutch (Protestant) descent. Her Canadian mother, originally from Newfoundland and Labrador, has English and Irish ancestry.