Ledley struck 20 minutes from time, in between Jason Puncheon's double, meaning a crushing knockout blow for Cardiff, who have just five games left to reach safety and avoid a quickfire Championship return.
And on this latest evidence, Ole Gunnar Solskjaer's team have little chance of recovering as Palace proved excellent value for their 3-0 win at a shellshocked Cardiff City Stadium.
It was Palace's second successive victory, arriving just seven days after they toppled title-chasing Chelsea, and confirmed a stunning continued revival under their Welsh-born manager Tony Pulis.
Palace, red-hot relegation candidates earlier this season, are now well clear of the drop zone. Cardiff, though, can only dream of such heights after they failed to deliver when it really mattered.
They remain firmly entrenched among the bottom three ahead of next Saturday's trip to Southampton, and there can be no excuses following an error-strewn and largely passionless effort.
Quite what watching Cardiff owner Vincent Tan made of it all is anyone's guess, with his team's brief stay in top-flight English football looking likely to end at some point during the next five weeks.
But Palace remain upwardly mobile under Pulis, who can surely start planning for next season safe in the knowledge that relegation is no longer an issue.
Former Palace favourite Wilfried Zaha, on loan at Cardiff from Manchester United, made the starting XI, while Mats Daehli and Kenwyne Jones also featured following last weekend's 3-3 draw at West Brom.
Palace manager Tony Pulis predictably retained the the team that rocked Chelsea at Selhurst Park seven days ago, which meant starts for ex-Cardiff players Ledley and Cameron Jerome.
Both teams struggled to make headway during an opening 20 minutes notable only for one shot blazed over the bar by Palace's Kagisho Dikgacoi and Cardiff's inability to keep possession in midfield.
Palace goalkeeper Julian Speroni was not troubled until the 24th minute, but he comfortably smothered a deflected corner low down at the post.
It was the cue, though, for Cardiff to increase the tempo, and with Daehli proving an industrious worker, gaps began to appear for Zaha and Fraizer Campbell.
But the home side rocked 14 minutes before half-time when Ledley proved the architect of their downfall.
The Cardiff defence allowed him far too much room out wide, and his ball into the box found Puncheon, who shot left-footed through a crowd of players past a stranded goalkeeper David Marshall.
Jerome then collected a yellow card after hacking down Jordan Mutch from behind, and team-mate Joel Ward quickly followed him into Phil Dowd's notebook for kicking the ball away high into the stand.
Cardiff's final ball left much to be desired, and there remained a nervousness about them that Palace readily took advantage of, especially given the comfort of a one-goal lead.
And Cardiff could find no way through before the break, with Palace in charge as they found themselves halfway towards collecting another three points.
Cardiff began the second period with Dowd waving away penalty shouts when Daehli went down, yet Solskjaer - despite a tepid first-half display from his team - had resisted any temptation to make early changes.
The first switch came after 57 minutes when Craig Noone, who had missed the last three games due to a groin injury, replaced a misfiring Zaha, while Pulis sent on Marouane Chamakh instead of Yannick Bolasie.
Noone injected some pace and purpose into Cardiff's play, and they looked a far more threatening outfit with him making angled runs from deep, ensuring that Palace's defenders came under sustained pressure for the first time.
But Palace weathered the storm, and grew in composure to dominate the closing stages.
Ledley rifled the ball home from close range after Marshall failed to hold a header low down - the ex-Celtic player did not celebrate - and Cardiff could find no way back as Puncheon completed their misery with a brilliant finish two minutes from time.