Justin von Tunzelman, 37, had been preying on the 29-year-old, who was taken by her friends to an office rest room to recover before being found by von Tunzelman, the London Evening Standard reported.
Prosecutor Lesley Jones told the Southwark Crown Court the financier "took advantage when (the woman) was unable to say yes or no to what was happening to her and he didn't think he would be caught".
The court was told during the week-long trial that a witness was "shocked" to enter the room later in the evening in July last year and find von Tunzelman and the woman naked.
Von Tunzelman claimed he had suffered drink-induced amnesia, but was found guilty of the sexual assault on the woman by a majority of 10-2.
The woman told the jury she had no recollection of von Tunzelman making advances to her during the evening, but other witnesses provided evidence of that nature.
The judge noted another woman had made allegations about von Tunzelman's "sexual conduct" in the past.
Von Tunzelman, who was educated at Auckland Grammar school and Otago University, was cleared of a second count of sexual assault of fondling the woman in the pub.
He was granted bail to appear for sentence on September 16.
