The music mogul said before Leona auditioned for X Factor's third series, he'd been concerned that the show wouldn't be able to attract and produce acts of a worldwide calibre.
Then came Leona who won X Factor and went on to become the show's biggest selling artist ever, shifting 20 million albums across the world and becoming a global superstar!
Speaking on BBC Radio 1, judge Simon said: "I think Leona made a huge difference because we were nervous until that point that we wouldn't find an artist that would break out of the UK.
"Then, I remember, about the third show when she was really good we started getting calls from American labels and were thinking, 'This girl could change the show forever' so I have a real soft spot for her."
Simon said he kept in touch with all his X Factor artists and spoke with them regularly.
As for the last X Factor star he spoke with? He'd only tease it was a member of One Direction but wouldn't name which one. Oh, Simon!
Simon quit the UK X Factor in 2011 to launch the US version of the show but he's returned to the UK judging panel this year for the 11th series.
He's joined by another returning judge, Cheryl Fernandez-Versini, long-time judge Louis Walsh and newcomer, Spice Girl Mel B.
X Factor kicked off its new series at the weekend. It airs Saturday and Sunday nights on ITV.
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Watch Leona's video for 'Bleeding Love' below: