Holt, who turns 33 next week, joined the Cobblers in June 2006 and has made over 200 appearances for the club. He will continue to play while also taking part in coaching.
Johnson said: "Everyone respects Andy as a person and he's got a great affinity with the football club.
"This is my appointment to introduce Andy into the coaching side. I still see him very much as a player, but I think he can be the buffer between me and the players in the dressing room.
"He will be learning all the time regarding the coaching side of things, but at the same time he has still got to think like a player."
Holt added: "It came a bit out of the blue but I'm really delighted to have this opportunity. Gary spoke to me on Wednesday and we had a good chat. I want to help the club in any way I can and this is the next step for me."
Source: PA
Source: PA