Whitley will be sentenced next month and faces between 16 and 60 years in prison.
Whitley's family broke into tears as the judge read his verdict Thursday afternoon. They left court without commenting.
"This victim was a 15-year-old girl who got into the car of a boy that she considered a friend, and she was brutally raped and she blamed herself for getting into that car. And we hope that, more than anything, that these findings of guilty will help her realize this was not her fault," said Cook County assistant state attorney Maria McCarthy.
"Clearly, we are disappointed. This was tough for everybody involved. I don't think anybody wins. He clearly has a supportive family," said Donna Rotunno, defense attorney.
In closing arguments, prosecutors said the girl's head was held down by Whitley friend and that "the defendant made her nightmare worse."
Prosecutor Kristin Piper went on to argue, "it's absurd that a 15-year-old girl would agree to have oral, anal and vaginal sex with boy she had never met."
Whitley testified that he had sex with the girl but she agreed.
Whitley's attorney argued, "we may not like the behavior these teens engaged in but that's not why we are here." Rotunna went on to say about the victim, "she's a liar. She lied to her mother, to police and to her friends."
But Judge Thomas Pecarotta said he didn't buy Whitley's testimony and believed the young woman's account of the evening in January 2010. "There's no doubt in my mind sex occurred and there's no doubt that it happened the way the victim said it did...when the defendant testified his testimony was unbelievable and poorly contrived."
The young victim, who testified in the trial, was in court with her parents as the verdict was read. They declined to comment and were clearly emotional as they left court.
Whitley is due back in court November 1.