Stewart Downing, James Morrison, Matthew Bates and Gary Liddle earlier this season also signed long term contracts.
Fifth teenage star pledges future to Boro
Stewart Downing, James Morrison, Matthew Bates and Gary Liddle earlier this season also signed long term contracts.
McMahon has made 15 starts this season (plus three sub appearances) and the Boro boss is delighted McMahon is staying on.
"This is great news for the club and good news for Tony," Steve McClaren told the club's official website. "He has made huge strides forward this season and has never looked out of place in the first team. There's still plenty of room for improvement, of course, but he can have a big future if he continues to work hard. It's great business for the football club too, because we are committed to developing and keeping our best young players. Tony has shown he is fully worthy of a new contract and will continue to play a big part for us in the months and years ahead."