The judge chose a black trenchcoat-style dress which featured three-quarter length sleeves, belt and asymmetric zip. Gold detailing set off the frock, which she paired with a red patent peep toe heels.
The Xtra Factor host Caroline Flack gave her a run for her money in the style stakes, arriving at the ExCeL Centre wearing a chic monochrome outfit.
She chose an oversized white blazer with the sleeves rolled up, black shorts and a pair of minimal strappy heels. A slick of bright red lippie and slightly dishevelled hair finished off the look.
Sharon Osbourne, meanwhile, went for a black suit, white shirt and orange pointed heels for the event.
The boys scrubbed up well too, with Gary Barlow opting for a navy blue polo shirt with black jeans, while Louis Walsh went for a shirt and blazer combo.
Take a look at some photos from the X Factor London auditions below.