Johnson has signed a two-and-a-half year deal at Sixfields and has called for the club to challenge for promotion next season.
Johnson said: "This is the right club at the right time for me.
"I`ve taken clubs up at this level before and I am really looking forward to getting going.
"It has been done very quickly but the chairman has pushed the boat out to get me in and I appreciate that.
"It isn`t necessarily the league you are in, it`s the club you are at and this club matches my ambitions."
Source: PA
Source: PA