Margaret Byrne has left her role as Sunderland chief executive officer admitting she made a "serious mistake" over her handling of the Adam Johnson affair.
Sunderland also expressed their regret that the young girl and her family were not only let down by Johnson's "despicable actions" but by the club itself.
Johnson was last week found guilty of sexual activity with a child having previously admitted two other lesser charges of grooming and sexual activity with a 15-year-old girl.
During the trial it was claimed Byrne received transcripts of police interviews, in which Johnson confessed to kissing a schoolgirl, and copies of his WhatsApp messages with her as early as May 4, 2015.
However, Johnson continued to play for Sunderland after this date and was only sacked by the club after he entered the two guilty pleas.
Sunderland released a statement defending their conduct, insisting Johnson had told them he would plead not guilty to all charges but questions were asked about how much Byrne knew.
And Byrne has now resigned, saying in a statement: "I recognise that, as CEO, my involvement with Mr Johnson and the decision to allow him to continue to represent Sunderland was a serious mistake."
Byrne also admitted to meeting with Orlando Pownall, QC who was subsequently appointed as Johnson's counsel and who sent her a note "which recorded, amongst other things, that Mr Johnson had kissed the victim and communicated with her."
Her statement then continues: "I did not share this information with anybody including the board of Sunderland AFC."
But Byrne is adamant she had no idea Johnson intended to plead guilty to any of the charges, adding: "Contrary to what has been suggested, I did not understand that Mr. Johnson intended to change his plea at trial or at all. I was astounded when he did plead guilty.
I accept that Mr. Johnson should not have been permitted to play again, irrespective of what he was going to plead. It was a serious error of judgment and I accept full responsibility for this."
Sunderland Statement In Full
"The Board of Sunderland AFC has today accepted the resignation of Margaret Byrne.
"Margaret, in her role as CEO, was responsible for the running of the club. She was also accountable for the actions taken by the club in relation to Mr. Johnson.
"Sunderland AFC acknowledges that Margaret's intentions have always been to act in the best interests of the club, however it has become clear through our own internal investigations that in this instance decisions have been taken by Margaret in error.
"Whilst swift and decisive action was taken to terminate Mr. Johnson's employment upon his guilty plea, decisions taken prior to this, including the decision not to suspend him for a second time pending the outcome of the trial, were wrong.
"In light of what has been acknowledged by Margaret as a serious error of judgment on her part, we have undertaken a full review of the club's decision-making processes to ensure that there can be no such mistakes in the future.
"Throughout this deeply regretful situation, we recognise that one devoted young fan and her family have been very badly let down, first and foremost by Mr. Johnson and his despicable actions, but also by the club they support. We are so very sorry for this.
"Mr. Johnson lied to the club; he also lied to our fans and they have every right to feel aggrieved by this. Lessons have been learned and we hope that the club and its fans can move forward from this together."