The music mogul rang Gary Barlow, Tulisa Contostavlos, Nicole Scherzinger and Louis Walsh from Miami and announced the categories they would each be guiding through the rest of the competition.
Cowell - who sat on the UK X Factor panel between 2004 and 2010 and is now a judge on the US version of the show - was first shown contacting Gary Barlow.
"Hello Simon. How are you?" he answered, before adding: "I'm alright, but give me the news quickly."
When Cowell told Barlow he was to mentor the over-28s, he replied: " Oh wow. Brilliant! That is good news to my ears."
Viewers then saw Walsh answer Cowell's call.
"Lou Lou, it's Simon. The decision we have made, god help them, is the groups," said Cowell. "Just be good at what you do, Louis."
"Oh, okay," replied Walsh, before putting the phone down. He then added: "I wasn't expecting the groups. But I'm happy. I'm going to work hard, and yeah, it's going to be great."
After they both revealed on camera that they would like to mentor the girls category, Scherzinger and Contostavlos found out that they had been assigned the boys and the girls respectively.
"I wasn't expecting that," said Scherzinger. "I'm stoked. I'm super stoked. I'm excited.
"In the boys group I think we have a real contender," she added. "A serious contender. Possibly the winner of X Factor."
Upon hearing the news, Contostavlos screamed and shouted: "Get in there! Thank you very much Simon, I won't let you down."
"I feel like in recent years, the overs has become a bit of a joke category and I really want to restore what the overs should be this year," added Barlow. "There are people over 28 with talent!"
Reflecting on her category, Contostavlos said: "Before, I said to myself there is no way I can win two years in a row, never going to happen. But my category is so strong that it's actually possible."
"I know there is something special in this category this year," added Walsh after he came to terms with Cowell's decision. "I think I have got a secret weapon in this category."
Watch the acts join the judges and special guest mentors for the Judges' Houses stage of the The X Factor next Saturday at 7.30pm on ITV1.
Photo gallery - The X Factor Final 24:
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