The 33-year-old midfielder has swapped west London for south London on a deal until the end of the season.
Mahon has not made a single appearance for Neil Warnock's table-toppers this term but he will be in line to make his Palace debut against Barnsley on April 2.
"I'm very pleased to have joined," he said.
"It's been stop-start for me this season and I need to be playing and in and amongst the first team.
"If I can give some input off the pitch as well as on it that would be great."
Source: PA
Source: PA