Phil, 57, showed he had absolutely no time for the conspiracy theorist, as Holly Willoughby looked on, baffled.
Mark features in Netflix documentary, Behind the Curve. He admitted he flies around the world to make people aware of his theory, but said he "flies straight".
And Phillip Schofield wasn't having any of it.
"Obviously there are thousands of questions that I'm sure you've been asked a thousand times... We can see that it's actually round. So how come it's not?" He said.
"I've been lucky enough to have flown on Concord and saw the curvature of the Earth. Are you saying that wasn't real?"
But Mark said: "I'm not saying you're lying I'm saying you want to see the curve."
Phil said: "I am a bit of a space nut and I have in the past looked at (NASA) pictures live on an app on my phone. So what am I looking at?"
We can see that it's actually round. So how come it's not?
And Mark said: "You're looking at CGI, something that started in the 1950s."
He believes the idea that the Earth is flat is a deliberate lie put about by governments trying to conceal the truth.
Mark even argued that astronaut Tim Peake, who has also been a guest on This Morning, was working for the government.
Viewers praised Phillip and Holly for their no-nonsense approach. One wrote on Twitter: "Credit to @schofe and @hollywills for holding this flat earth man to account @thismorning.
A second wrote: "I feel so normal and grounded (pun intended) after the flat earth interview. Wowsers."
But another said: "Still giving the flat earth idiot airtime? Why?"
— Amy Allen (@amyjade5150) February 11, 2020
I feel so normal and grounded (pun intended) after the flat earth interview. Wowsers.
— 73sgg (@73sgg1) February 11, 2020
Still giving the flat earth idiot airtime? Why?
— Marion Tinney (@mazjt) February 11, 2020
Others found Phil's responses hilarious.
— Splash Music (@splashmusicuk) February 11, 2020
— Ivana (@irichards10) February 11, 2020
— Emma Staple (@singalot_staple) February 11, 2020
Was Phil right to give the guest a hard time, or should he have gone easier on Mark? Go to our Facebook page @EntertainmentDailyfix and leave your comments.