The Welsh midfielder’s second-half goal was enough to snatch the victory for Alan Pardew’s men, but the home side will feel aggrieved, as they had the chances to earn at least a point.
The Foxes had the better of the first-half opportunities, with David Nugent coming closest to breaking the deadlock but his header came back off the post.
Yaya Sanogo was introduced after the break by Pardew, and he gave the visitors more impetus.
And that momentum resulted in Ledley giving the visitors the lead they held on to, and continued their improvement under their new boss.
The Foxes were showing signs of the strain getting to them, with boss Nigel Pearson involved in a??touchline fracas with Palace midfielder James McArthur.
After their fifth win in six under Pardew, the Eagles remain in 13th, but Leicester have lost three in a row, and remain rooted to the bottom of the table.
Palace started the match on the front foot, and through Zaha, they looked a real danger on the counter. The former Manchester United man was certainly at his trickiest, but he could not produce the goods.
For all their probing, Palace looked a little vulnerable at the back, and were fortunate not to concede.
Julian Speroni got down to a Riyad Mahrez drive to push the ball out, with David Nugent unable to capitalise on the rebound.
Nugent then had the chance of the first-half, but his header from a dangerous corner came back off the post, with Speroni well beaten this time.
The introduction of Yaya Sanogo at half-time gave the host added impetus, and it wasn???t long before they were in front.
A left-wing corner picked put another Palace substitute, Brede Hangeland, at the back post, who headed back into the six yard box, and Ledley, on the run, headed past a stranded Mark Schwarzer.
Almost immediately Ledley could have turned from hero to villain, as his woeful backpass was seized upon by Nugent, but the former Everton forward wasted the chance.
Speroni was called upon on several other occasions, with several fine saves thwarting substitute Jamie Vardy.
McArthur???s clash with Pearson was quickly diffused by referee Lee Mason, with Pearson showing signs the pressure is getting to him, and after another home defeat, the Foxes are starting to creep closer to relegation.