The England international took his total for the season to 18 including an incredible nine from the penalty spot. It proved to be Palace's first double of the season and this afternoon's win gives them a four-point cushion over Southampton who are sitting just below them.
For Birmingham they are having a rough time away on their travels, this being their fifth defeat on the trot.
The first half was shaded by Palace although Birmingham could look back on a couple of missed chances. After only five minutes Jermaine Pennant's outswinging corner from the right was met by England defender Matthew Upson, but with so much time to place his header he instead directed it straight into the arms of Gabor Kiraly.
At the other end, Johnson was close to getting a penalty after he went down following a challenge. Referee Phil Dowd turned down the appeals. It was a very close decision.
On 25 minutes, Julian Gray did well on the left, neatly tapping inside before releasing Clinton Morrison. The Republic of Ireland hitman took the ball in his stride, pushed forward a few yards, before firing at Kiraly, but fortunately for the home side the ball rolled past the outside of the post.
Palace were beginning to win the midfield battle with skipper Michael Hughes in particular prompting and probing everywhere.
Johnson was a threat every time he was in the action, but it was midfielder Tom Soares who was involved in the build-up that lead to the penalty. His strong run into the box ended with Upson mistiming his tackle and this time referee Dowd pointed to the spot.
Johnson shaped up with a fierce spot-kick which flew slightly to the right of Maik Taylor who had no chance. But Mr Dowd ordered the kick to be retaken after stopping encroachment and Johnson had to return from the celebrations to plant the ball back on the penalty spot.
It mattered not as this time Johnson firmly stuck a low shot to Taylor's left and neatly into the corner.
After the break, City had the better of the early exchanges. Passes were being strung together and Palace for the first time were pushed on to the back foot as they lost the initiative.
But the Eagles' defence was in solid shape with Gonzalo Sorondo and Fitz Hall faultless at the back and when called upon Kiraly showed a safe pair of hands, competently cutting out the steady supply of crosses from Gray and Pennant.
Birmingham were enjoying their best spell and Upson provided a flying header from Pennant's cross which Kiraly did brilliantly to save.
Palace struggled to press the Birmingham rearguard until on 60 minutes Dougie Freedman cleverly cut into the area, but just 15 yards out he placed his shot tamely and Taylor was able to make a routine save.
Nevertheless, this break was just the impetus Palace needed. Seven minutes later referee Dowd awarded Palace another penalty.
It all started with Wayne Routledge passing inside to Hughes who cleverly slipped the ball into the path of Johnson. The Palace marksman turned Upson forcing the defender to make a desperate challenge. Dowd was just a few yards away and judged the challenge illegal as Johnson fell to the ground.
Johnson, cool as ever, pumped his penalty straight down the middle with Taylor diving in vain to the right.
The remaining 20 minutes or so turned in Palace's favour. Birmingham offered little and defensively the Eagles were outstanding while Mikele Leigertwood and Hughes continued to boss the midfield.
Palace were happy to contain and they did just that to claim three precious points.
Man of the match: Andy Johnson - Was a constant threat to his former club throughout the afternoon. He never stopped running and his spot-kicks were perfect to put Palace on their way to a vital win.