The 21-year-old leaves Middlesbrough for good after spending last season on loan with the Cobblers and has signed a two-year contract.
"John is young, hungry, determined and ambitious," said Ian Sampson.
"We know there were a lot of clubs interested in him so as soon as we heard he could be available we moved quickly to bring him to the club.
"I am sure the move here will prove good for him and we are pleased to have made another major signing."