The former Another Level and Celebrity Big Brother housemate, 35, had claimed he was acting in self defence when Sophia was hit on the nose during an altercation at his parents' home in January.
He insisted it was accidental.
But District Judge Andrew Sweet at Croydon Magistrates' Court said: "I am satisfied that Ms Cahill told the truth, and that Mr Bowers was very angry. I am satisfied that he assaulted her in the manner she described."
Dane was bailed and will be sentenced on 15 October. The judge added: "Bearing in ,ind the nature of the assault, I'm not ruling out the possibility of a prison sentence."
Former pop star Dane had separated from former beauty queen Sophia, 32, at the time of the incident but they had continued to live together at his parents' home in Croydon, South London. The court heard that the row began when he arrived home after a night out with glitter on his face, prompting Sophia to tell him he was "taking the piss".
According to ITV, she told the court: "I had caught him in bed with somebody (before) - that obviously gave us trust issues. We were still living together and we said that if we are still living under the same roof we would not do anything to disrespect each other and do anything with other people.
"He had been out all night and obviously I jumped to conclusions because of other things he has done in the past."
Sophia claimed the assault happened after she quizzed him about the glitter.
Dane had said he was at a circus-themed event trying to help a friend get a job.