The midfielders, who move from Blackpool and Celtic, respectively, round off a busy day of transfer business at Selhurst Park, as Tony Pulis added five new faces to his squad
As reported by Goal earlier in January, England Under-21s international Ince - who had been granted permission to talk with clubs after his father, Paul, was dismissed by the Seasiders - was wanted by a host of Premier League sides, including Palace.
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Blackpool, who have received Stephen Dobbie on loan in return as part of the deal, thanked Ince for his contribution since joining from Liverpool in 2011, having scored 31 goals in 100 Championship appearances.
A statement read: "Thomas Ince has left Blackpool and joined Crystal Palace on loan until the end of the season. The winger had been granted permission to speak to Premier League clubs this week and has joined the Eagles on deadline day.
"Blackpool would like to thank Thomas for his efforts and wish him the best of luck in the future."
Ledley, meanwhile, has been a key part of a Celtic side who have raced to an 18-point lead in the Scottish Premiership and the midfielder, who boasts Champions League experience, signs for an undisclosed fee.
The pair join three further signings, Jason Puncheon, Wayne Hennessey and Scott Dann, on what has been a busy and productive deadline day for Tony Pulis.