The Eagles run of one win in 12 cost Steve Kember his job on Monday and defender Kit Symons was told to get on with it as caretaker boss.
Preston went in front from David Healy on 51 minutes before Johnson deservedly put Palace on level terms 6 minutes from the end.
The first half was a none event. Healy had a shot at goal then took a deflection, while at the other end, the best chance fell to Ben Watson when with only Jonathan Gould to beat, his shot hit the underside of the bar and went out.
Shaun Derry also fired over the bar for Palace, but generally it was dismal entertainment.
The second half got better and within six minutes Healy fired the visitors in front with Palace wide open after poor clearance up field.
Healy was played onside and cooly slotted the ball past Thomas Myhre.
Within ten minutes, Preston had a massive let off with Gould escaping what otherwise would have been a certain red card.
Shaun Derry's pass put Andy Johnson clear and the Palace hit man was caught by Gould as the keeper left his goal.
But Referee Ian Wiley, a mile away from play, booked Johnson for a dive and Gould remained on the pitch.
Palace were now pushed into more urgency and Danny Butterfield was unlucky with two good efforts, while Johnson was unable to beat Gould after being left with a one on one.
But Palace got their reward six minutes from time, when a long up-field pass was flicked on by Neil Shipperly and Johnson raced on to finish with a low drive past Gould.
Palace were going all out for the three points but that meant Preston were left with space and indeed chances to snatch victory. Healy and Richard Cresswell missed good chances with Myhre making some splendid stops.