The former Spice Girl threw her drink at her fellow "America's Got Talent" judge Tuesday after he made a joke about her wedding night.
The incident occurred during the live show Tuesday night as magician Demian Aditya attempted an escape trick involving a wooden box, fire and steel stakes.
The trick didn't come off, leaving host Tyra Banks to explain that there had been a technical issue that was not Aditya's fault.
"I kind of imagine this would be like Mel B's wedding night," Cowell joked. "A lot of anticipation, not much promise or delivery."
Brown didn't take kindly to the crack and tossed the contents of her cup at Cowell even before he got the punchline out.
She then walked off the set.
"Mel B is out," Banks told the audience. "This is live TV, y'all."
The show's official Twitter account tweeted that Brown had left the building.
It's not the first time Brown has doused Cowell.
"He should know, if you say something inappropriate he's gonna get like stuff thrown at him," she told "Entertainment Tonight"
afterward. "In a friendly way. I made sure there was no ice in it this time."
Brown returned during the commercial break.
"I told him he was an a**hole and that was it and he goes, 'Oh, whatever,'" Brown told ET.
Brown is currently going through a contentious divorce with her husband of 10 years, Stephen Belafonte.