Johnson struck after goals from Stephen McPhee and Richard Garcia had put Hull on course for a fourth win in five games.
However, Cardiff were unlucky to go in at the break a goal down after scoring through Steve Thompson. Kasper Schmeichel also saved a first-half penalty from Dean Windass.
Hull got off to a flying start when McPhee started and then finished a lightning attack by sweeping home Garcia's cross from the right.
The lead only lasted for a matter of seconds though as the impressive Gavin Rae cut in from the right and Thompson stroked the ball into the bottom right corner from the edge of the area.
Cardiff dominated the remainder of the first half with Hull goalkeeper Boaz Myhill pushing Joe Ledley's angled shot against the upright before Ledley went close with a header from six yards.
However, with the away side in control, Darren Purse clattered into Fraizer Campbell. Windass was unable to convert the penalty though, with Schmeichel saving with a dive to his right.
Seconds before the break Schmeichel flapped at a cross from Damien Delaney and Garcia converted with a first-time volley.
Myhill produced a stunning save to deny Thompson's powerful header early on in the second half and then the same player snatched at a shot from eight yards after being played in by Steve MacLean.
Myhill again produced heroics to block MacLean's shot after the Hull keeper had dropped the ball under pressure.
Three minutes into stoppage time though Johnson scored with a brilliant long-range shot to save a point for Cardiff.
External Reports
The Hull City Fanzine
Yorkshire Post
South Wales Echo