Jason Puncheon also scored twice at the Cardiff City Stadium on Saturday as Tony Pulis' side gave their Premier League survival hopes a huge boost.
With only five points separating the two sides heading into the game, Pulis had insisted his side would do Cardiff no favours, despite the Welshman having watched the club during his youth.
His words certainly bore truth in a spirited first half as a fierce drive from Puncheon saw Palace lead 1-0 at the break.
The hosts pushed for a leveller in the second half, but they were caught out when Marouane Chamakh – returning from a month-long hamstring lay-off – saw a header parried into the path of Ledley who duly netted the follow-up on his first return to Cardiff since leaving for Celtic in 2010.
Puncheon added further gloss in the closing stages with a superb curling effort from 20 yards, as Palace moved seven points clear of the relegation zone to 14th, while Cardiff face an uphill task to avoid the drop with a six-point gap to 17th-placed Norwich.
The visitors settled the better of the two sides and Kagisho Dikgacoi firing the first sight of goal in the fourth minute high and wide.
After a lacklustre opening, Cardiff began to gain a foothold in the game, with ex-Palace winger Wilfried Zaha giving his former employers a timely reminder of his potential with a series of promising runs that failed to yield opportunities.
Cardiff's improvement resulted in their first effort on target after 24 minutes, Fraizer Campbell ghosting between Palace's centre-backs to head Jordon Mutch's cross from deep and force a smart low save from Julian Speroni.
However, the home side's failings in defence were once more evident in the 31st minute as Puncheon opened the scoring, shortly after wasting a similar opportunity.
Good work from Ledley led to Puncheon working room on the right side of the penalty area before he drilled a fierce left-foot strike into the near corner.
The second half started in a similar fashion to the first with Palace asking the questions, and Ledley bent a shot wide from 18 yards in the 51st minute.
From the next attack Cardiff had tentative claims for a penalty waved away by Phil Dowd when Mats Daehli went to ground when challenged by Dikgacoi.
Cardiff forced a scramble when Speroni spilled Craig Noone's cross in the 65th minute, but Palace survived and duly added the second.
Chamakh leapt highest to meet a free-kick that was expertly saved by David Marshall, yet he could do nothing to prevent Ledley tapping home the rebound.
Puncheon found the top corner with two minutes of normal time to go, adding a dominant air to the result which could ultimately seal Palace's survival and Cardiff's fate.
