First-half goals from Shaun Harrad and Guillem Bauza had looked set to hand the relegation-threatened Cobblers a first win in 16 games. But Scott Donnelly's neat finish and Johnson's late header earned the Chairboys a point to keep them third in the npower League Two table.
Northampton bossed the first half and Harrad put them ahead from the penalty spot after 20 minutes, drilling the ball down the middle after Scott Donnelly had barged over Paul Rodgers in the box.
Wycombe midfielder Lewis Montrose blasted a 30-yard half-volley narrowly over the crossbar before Bauza capped a swift counter-attack to double the Cobblers' lead in the 37th minute.
Bauza raced clear onto Josh Walker's pass to slot the ball low past keeper Nikki Bull after good work on the right by Harrad. Harrad was denied one-on-one by Bull shortly after the break before Donnelly pulled a goal back for Wycombe just seconds later, stealing in behind Dean Beckwith to fire the ball inside the near post.
Cobblers keeper Steve Collis saved well from Donnelly, substitute Stuart Beavon and Stuart Lewis before Johnson headed in Donnelly's corner eight minutes from the end to seal the Chairboys' comeback.