The defender made his first-team breakthrough this season, appearing for the first time in the 2-1 home win over West Ham in September.
McNair has gone on to make 10 Premier League appearances in all, seven of which have come as a starter, as well as two outings in the FA Cup.
The 19-year-old has been in and out of the side as Louis van Gaal continues to juggle his options at the back, but Tuesday's announcement has tied down his future for at least a further two years.
Van Gaal said: "Paddy has risen through the ranks to become an integral part of the first team squad. He has all the attributes to develop into a top player.
"Promoting youth players is very important both to myself and to the club and I am delighted Paddy has signed a new contract."
McNair added: "To make my debut for this great club earlier this season was a dream come true. I'm pleased to have signed a new contract.
"I have come through the academy here and I feel I am improving my game all the time with the help of the manager, the coaching staff and my team-mates."
United welcome Burnley on Wednesday as they seek to enhance their bid for a return to the UEFA Champions League.
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