The Cobblers have lost just twice in their last 16 matches in League Two and kept eight clean sheets.
Johnson told BBC Radio Northampton: "The manager and [coach] Malcolm Crosby turned it around, so credit to them.
"When we were struggling, they did a lot of work with the back four, taking us away from the rest of the team. It's obviously worked well."
The 21-year-old says the players are encouraged to see manager Ian Sampson sign a three-year deal at Sixfields.
"He's done a great job, so he deserves an extension," added Johnson.
"Those players [whose contracts are expiring] know the manager is going to be here, so it's an extra bonus for them."
Johnson is on-loan from Middlesbrough until the end of the season.
Source: BBC Sport
Source: BBC Sport