He was cleared of a second charge relating to another alleged sexual act with the girl during a hearing Wednesday at Bradford Crown Court in the north of England.
The 28-year old had previously admitted to grooming a girl under the age of 16 and one charge of sexual activity.
Johnson, who faces jail, has been granted bail with sentencing to take place at a later date.
The charges related to an incident in Johnson's car on January 30, 2015, after he met up with the girl.
Police said Johnson initially said he was unaware of the girl's true age and claimed he only kissed her. He later admitted to the allegation of grooming a child under 16.
The girl told the court that Johnson put his hands down her pants and she performed an oral sex act on him. The charge related to the girl performing oral sex on him wasn't proved and Johnson was cleared of that.
Exploiter
"Adam Johnson was idolized by many young fans for his football career," said Gerry Wareham of the Crown Prosecution Service. "However, he chose to exploit that fame to actively pursue a 15-year-old girl for his own sexual gratification, in full knowledge of her age.
"Adam Johnson knew that what he was doing was wrong and went to great lengths to hide it," added Wareham.
"He encouraged his young victim to delete any traces of their online contact and, when she reported what had happened to police, he initially claimed to be unaware of her true age and told police that he had only ever kissed her.
"The victim in this case has endured months of abuse and personal attacks online. Had Adam Johnson wished to do so, he could have spared her further alarm and distress."
Johnson's lawyer did not immediately respond to CNN's request for comment.
England international
Johnson, who last played for England in August 2012, was arrested in March 2015 and was initially suspended by Sunderland before returning to the team a fortnight later.
Sunderland, which had initially declined to comment on the grounds the case was ongoing, then chose to sack the midfielder last month in light of his guilty plea. Following Wednesday's verdict, the club sought to clarify its stance, saying had it known all along that Johnson would plead guilty at any time it would have terminated his employment "immediately."
"This has been an extremely difficult time for all involved," Sunderland said in a statement on its website. "The victim and her family have endured an unimaginable ordeal in the last 12 months and we trust that they will now be allowed to move on with their lives without further intrusion or public scrutiny."
Sportswear firm Adidas also confirmed to CNN last month that it had terminated its sponsorship deal with the player following his guilty plea.
Johnson began his career at Middlesbrough before moving to Manchester City and then to Sunderland.
While playing for City between 2010 and 2012, Johnson won the Premier League title and the FA Cup.
