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 Ashkenazi Jews (the original family surname being "Medvedof"), who arrived in the United States from the Russian Empire. Her paternal grandmother was of Dutch (Protestant) descent. Her Canadian mother, originally from Newfoundland and Labrador, has English and Irish ancestry.

Maddow has said her family is "very, very Catholic" and she grew up in a community that her mother has described as "very conservative". Maddow was a competitive athlete and participated in high school volleyball, basketball, and swimming.

Referring to John Hughes films, Maddow has described herself as being "a cross between the jock and the antisocial girl" in high school. She is a graduate of Castro Valley High School and attended Stanford University. While a freshman, she was outed as a lesbian by the college newspaper when an interview with her was published before she could tell her parents.

She earned a degree in public policy at Stanford in 1994. At graduation, she was awarded the John Gardner Fellowship. She was the recipient of a Rhodes Scholarshipand began her postgraduate st…

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