Fans were surprised to see them on the screen together in the first place because of their infamous feud, which saw them fight at an awards ceremony in 2004.
Jeremy kicked off the show by saying: “The first contestant is someone I’ve had a few disagreements over the years.”
He added he’d decided to put their differences aside after getting into the festive spirit, but couldn’t resist cheekily adding: “It’s the simply awful human being that is Piers Morgan. “
Piers came out laughing and his back: “It did not say that on the auto cue.”
Jeremy said: “It did, you should see what I’d written.”
Taking to Twitter, they shared their thoughts, with many noting that they didn’t believe their apparent banter was all that light-hearted.
“Think Piers and Jeremy genuinely hate each other,” one said. “The banter isn’t even disguising it.”
Another wrote: “Hang on… Jeremy Clarkson and Piers Morgan in the same room?”
A third tweeted: “Morgan v Clarkson. The rematch on ITV shortly.”
Someone else said: “Watching Piers Morgan and Jeremy Clarkson on Millionaire is so uncomfortable.”
And a fifth added: “Have I died and gone to hell, or are Piers Morgan and Jeremy Clarkson actually on TV at the same time?”
The duo went back and forth at the start of Piers’ go, with Jeremy making a dig as he read out his introduction.
Piers protested that it “did not say that on the autocue”, however Jeremy quipped that what he had written “was way worse”.
Soon, Piers said: “I don’t watch this show. I used to watch the old one when it was by a proper stand up character,” meaning the old host Chris Tarrant.
Jeremy then taunted Piers, telling him: “You’ll never make it but you’ve got 15 questions to get to £1million.”
It turns out Jeremy was right, and Piers ended up leaving with just £1,000 for charity.
On Christmas Day’s Good Morning Britain, Piers previewed his time on Millionaire and brought up his feud with Jeremy.
He said: “For reasons that still baffle me I agreed to go on Who Wants To Be A Millionaire hosted by a man who’s punched me repeatedly in the head.”
Piers went on: ““Jeremy Clarkson, who I had a long-term feud with, who actually broke his finger hitting my head. That’s how hard my head is.”
Who Wants To Be A Millionaire continues Sunday at 9.15pm on ITV