The big skipper smashed home from close range with just three minutes to go to maintain his side's unbeaten start to the season.
Wanderers had dominated for most of the 90 minutes and will feel that justice was done.
But they didn't have it all their own way, and for long periods in the second half Carlisle looked likely to end Wycombe's 18-match run.
Despite Tommy Mooney pulling the strings for the away side and Kevin Betsy running the Carlisle defence ragged, Wycombe created few clear cut opportunities.
Mooney, with several long range efforts, and Joe Burnell both went close but Carlisle keeper Keiren Westwood had surprisingly few saves to make.
Carlisle, themselves on an unbeaten run of five games, came out fighting in the second half with substitutes Adam Murray and Michael Bridges changing the game in Carlisle's favour.
Striker Karl Hawley almost gave the Cumbrians the lead just before the hour when his slick header ballooned into the air and dropped onto the Wycombe crossbar.
Then Murray had a great dipping volley saved by Ian Turner as Carlisle pushed on, with the Brunton Park crowd effectively a 12th man for the home side.
Hawley was again unlucky when his 25-yard drive looped off Johnson's foot only to land inches wide of the post.
But John Gorman's men weathered the storm and hit the woodwork themselves through Betsy before Johnson intervened.
Clint Easton's corner was rammed against the post by Charlie Griffin and, with the ball flying around the box, Johnson stretched to smash the ball in to the delight of the visiting fans.
Carlisle then pushed forward but the Wycombe defence held firm to give them a richly deserved three points and remain clear at the summit of League Two.