Jurors at Bradford Crown Court found the former Sunderland FC winger guilty 10-2 after Judge Jonathan Rose said he would accept a majority verdict.
The disgraced sportsman was cleared on a second count – oral sex with a child – after a trial that last more than two weeks.
Johnson, 28, admitted grooming a 15-year-old girl and sexual activity with her, relating to kissing her in his Range Rover, but denied the two more serious charges of sexual activity with a child, one involving oral sex and another involving digital penetration. He was found guilty of the latter charge.
The charges against him arose from a meeting between Johnson and the girl in his car in County Durham on 30 January last year.
Judge Rose said a custodial sentence is “the almost inevitable outcome” but granted Johnson bail until the hearing, which will be held at a later date.
There was no obvious reaction to the verdict from Johnson, who was sitting in the dock with two security guards.
The judge said his preliminary view was that the case falls into the category of a five-year prison sentence with a range of four to ten years.
He said: “The defendant must understand there is a very high probability of a significant custodial sentence.”
In a statement issued after the case, Sunderland FC said it “refuted” any suggestion the club knew all along that Johnson was intending to change his plea just before his trial so he could continue to play for them, and that the club may have been involved in tactical discussions about the plea.
The club said it was not advised in advance that Johnson would plead guilty to any offence.
“Had the club known that Mr Johnson intended to plead guilty to any of these charges, then his employment would have been terminated immediately,” the statement said.
“Indeed, upon learning of the guilty plea on 11 February, the club acted quickly and decisively in terminating Adam Johnson’s contract without notice.
“The club did not give evidence either for the prosecution or the defence in this case.
“It was therefore not present in court when it is understood that a suggestion was made that the club knew all along that Mr Johnson was intending to change his plea just before trial to enable him to continue to play football for the club and that the club may also have been involved in tactical discussions about the plea.
“This is utterly without foundation and is refuted in the strongest possible terms.
“The club never placed any pressure or demands on Mr Johnson to play football during this process.”
Just before he left the dock, Judge Rose told Johnson that being released on bail meant he could get his “affairs in order”.
He said: “You can say goodbye to your daughter. A prison sentence will mean you will not see her for some time.”
Explaining his bail conditions, the judge said Johnson would be “well advised to stay well away from social media”.