Recently-married Clooney has filmed a special spin-off sketch for the hit period drama for ITV’s annual Text Santa appeal, to be screened later this month.
In the first images to be released from the programme, the pair - who previously worked together on the film The Monument Men - are seen with a clapperboard in front of a Christmas tree.
Clooney married British human rights lawyer Amal Alamuddin in a private ceremony in Venice in September.
His co-stars have previously said they were unsure he would be appearing until he turned up on set for the drama, which is filmed at the Highclere estate in Hampshire.
Actress Laura Carmichael has also spoken about how Clooney is seen planting a kiss on the Dowager Countess, played by Dame Maggie Smith, during his appearance, which prompts her to swoon.
Ms Carmichael said: “George plants a kiss and Maggie ends up on the floor, it’s wonderful. He was hilarious.”
Downton Abbey is set to be the centrepiece of ITV’s Christmas schedule.
Elizabeth McGovern. who plays Lady Cora, has reassured fans that there will no be shock deaths in this year’s Christmas special.
She also hinted that the festiv special will end on an upbeat note.