Mahon still has a couple of years to run on his Blackburn contract and Bluebirds boss Lennie Lawrence ruled out any permanent deal at Ninian Park for the player because of the finances involved.
"We don't know at the moment where we are in regards to next season, so he's going back," said the Cardiff boss.
"Alan has done very well for us and sparked things off nicely in the opening weeks he was here.
"But he has found it difficult recently in that he didn't know whether he was staying or leaving. We wish him all the best."