The Armagh native step down after it emerged that she was aware of Johnson's actions before the start of his trial
Sunderland say that they have accepted the resignation of chief executive Margaret Byrne in the wake of the Adam Johnson scandal.
Byrne has been forced to step down after it emerged that she was aware of Johnson's actions before the start of his trial on charges of sexual activity with a 15-year-old girl.
In a statement, the club say that Byrne showed "a serious error of judgment" by choosing not to suspend the player from lining-out for the club after she became aware of his actions.
The statement adds that the club is very sorry that one of their supporters and her family have been badly let down, "first and foremost by Johnson and his despicable actions", but also by the club.
In a statement, the club said: "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."