The former England footballer was cleared of another count in which she was alleged to have performed a sex act on him.
The judge Jonathan Rose told Johnson a substantial custodial sentence was “an almost inevitable outcome” when he is sentenced later this month at Bradford crown court.
In a powerful statement issued after the verdicts, the victim – a lifelong Sunderland fan – said she felt “used and let down” by her former idol.
“The last 12 months have been horrendous and there have been times when I’ve wanted to hide away from the world. There have been times I haven’t felt able to face people,” she said.
“There have been times when I’ve tried not to show people how upset I am, but sometimes it hasn’t been possible and I’d just cry. I’ve felt so broken.
“I’ve been in some very dark places over that time and I thought the trial and giving evidence, having my say, would give me closure. But it didn’t. It put me back into the same dark places and I felt worse than I’ve ever felt before. This was because I still didn’t feel believed.”
Judge Rose told Johnson as he was granting him bail that he should use the opportunity to “say goodbye to your daughter” because “you will not see her for some time”.
Johnson’s daughter was seven weeks old when he was arrested exactly a year ago.
The judge said his preliminary view was that the case fell into the category of a five-year prison sentence with a range of four to 10 years.
The judge said: “You must understand when you return here at the end of March that it will be to receive a substantial prison sentence. Do you understand that?”
Johnson nodded in the glass dock, flanked by two officers. The judge told Johnson he must observe a 7pm to 7am curfew and also said to him that “you would be well advised to stay well away from social media”.
Johnson left the court dock without answering a journalist who asked whether he would apologise to the victim. A minder carried a black hold-all bag packed in case Johnson was jailed immediately.
In chaotic scenes as he left court, Johnson’s father, Dave, lost his spectacles in a scuffle in which he appeared to grab a BBC television reporter by the arm while being buffeted towards a waiting vehicle.
The jury of eight women and four men deliberated for nine hours and 42 minutes after which they found Johnson guilty of one count of sexual activity with a child.
They returned with a majority of 10-2 after failing to reach a unanimous verdict.
The jurors convicted the 28-year-old just half an hour after acquitting him on another count of sexual activity with a child.
In her statement, the girl said she hoped Johnson “realises the hurt and damage he has caused” and that she would now be allowed to rebuild her life.
“The last 12 months have been horrendous and there have been times when I’ve wanted to hide away from the world. There have been times I haven’t felt able to face people.
“There have been times when I’ve tried not to show people how upset I am, but sometimes it hasn’t been possible and I’d just cry. I’ve felt so broken.
“I’ve been in some very dark places over that time and I thought the trial and giving evidence, having my say, would give me closure. But it didn’t. It put me back into the same dark places and I felt worse than I’ve ever felt before. This was because I still didn’t feel believed.”
The footballer showed little reaction from the dock as the verdict was delivered. His sister sat with her head bowed in the public gallery.