The England international has joined until January and provides cover for the injured Yassin Moutaouakil.
Addicks boss Alan Pardew believes Mills' experience can only help his side's bid for an immediate return to the Premier League.
"The reason we brought Danny Mills to the club is we feel we are lacking a bit of organisation at the back and at times, a little bit of experience," said Pardew.
"We also needed someone, I think, who is aware what this division is about."
Mills goes straight into the squad for Saturday's vital away trip to Crystal Palace.
Another player believed to be SE7 bound is Reading defender Sam Sodje who will sign a one-year loan deal tomorrow. Robert Earnshaw remains linked with a transfer deadline move.