"He was a big success here last season. He knows all about us, we know all about him and he fits the bill," said manager Ian Sampson.
"He is a good defender and he is a goal threat for us, both at set pieces and in open play."
Johnson is the Cobblers' fourth summer signing, after Tadhg Purcell, Nathaniel Wedderburn and Alastair Slowe. Former Coventry City defender Marcus Hall is on an extended trial.
I am sure the move here will prove good for him and we are pleased to have made another major signing
Ian Sampson
Sampson added: "John is young, hungry, determined and ambitious.
"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."
Source: BBC Sport
Source: BBC Sport