Now, the duo and co-judges are pretty good pals, but in the past things weren't always so amicable. Reflecting on her changed friendship with the music mogul, Sharon admits she used to be a bit "prickly" with Simon and claimed the pair didn't always see eye-to-eye thanks to their "big egos."
Speaking to The Sun, Sharon explained: "We first worked together 13 years ago when I was all prickly and had a big ego - so did Simon. You change. Now I say to him, 'Shut up,' and he goes to me, 'Shut up.' Then you have a laugh about it.
"Back then, I was like, 'Don't you know who I am?' It was the same with [Simon]. We both had egos, but you grow up. It's like how people say if you are the same person you are at 20 when you are at 50, then you've wasted your life."
Simon and Sharon first appeared back on The X Factor panel together way back when it first started in 2004. Sharon was on the show for four series (the first three with Simon and Louis Walsh, Dannii Minogue joined in the fourth), before leaving in 2007 and being replaced by Cheryl the following series.
She did return to The X Factor in 2013, but only for one series when Simon hadn't been on the judging panel.
Now though, Simon and Sharon have been reunited. She returned to The X Factor for a third time, along with Louis Walsh (who took a year's hiatus from the show) and Nicole Scherzinger and said she was "overjoyed" to be back.
Speaking before filming got underway for this year's series, Sharon said: "I'm overjoyed about going back to The X Factor, sitting with Lou, Lou, gorgeous Nicole and naughty Simon."
Now, Sharon is preparing for the Judges' Houses stage of the competition and she has revealed that she would sign up for The X Factor next year, if Simon wanted her too.
In an interview with Daily Star, Sharon claimed: "If Simon said, 'I'd like you to come,' and ITV agreed, then yes."
Sharon is currently mentoring the overs and fans will see her whisk her acts off to LA, to audition for the live shows in her new home. Among her acts will be rapper Honey G, after Sharon chose to reinstate her into the competition to replace Ivy Grace. Ivy was forced to withdraw from the show, following visa issues.
The X Factor continues Saturday 1 October at 8pm on ITV.
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