Middleton was born in Royal Berkshire Hospital, Reading, the first of three children of Carole Elizabeth (née Goldsmith; born 31 January 1955) – an air hostess – and Michael Francis Middleton (born 23 June 1949), a flight dispatcher for British Airways. Michael and Carole had married on 21 June 1980 at the parish church in Dorney, Buckinghamshire. Middleton is of overwhelmingly English descent, but with a few distant Scottish and French Hugenot ancestors. Middleton's paternal family came from Leeds, West Yorkshire, and her great-grandmother Olivia was a member of the Lupton family, who were active for generations in Leeds in commercial and municipal work. Carole Middleton's maternal family, the Harrisons, were working class labourers and miners from County Durham. Middleton has two siblings, Philippa "Pippa" Charlotte and James William. Pippa Middleton, a graduate of the University of Edinburgh, has received press coverage since her sister became famous, with focus on her relationships and lifestyle.
In 1987, the Middletons founded their own company, Party Pieces, a mail order firm that sells party supplies and decorations. They have since become millionaires.
She was raised in Bucklebury, Berkshire, and attended St. Andrew's School in Pangbourne, followed by Marlborough College. She attended the University of St Andrews, where she met Prince William, and graduated with a 2:1 (Hons) in history of art.