London - Sunderland's chief executive Margaret Byrne announced her resignation on Tuesday after admitting to making "a serious mistake" in her handling of the Adam Johnson affair.
"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 said in a statement.
Sunderland allowed Johnson, 28, to continue playing for them for nearly a year despite knowing that he had admitted to kissing a 15-year-old girl.
He was sacked by Sunderland last month after he admitted to grooming the girl and one count of sexual activity with her. He is awaiting sentencing after being convicted last week of a more serious sexual activity offence.
During the trial, the court heard that Johnson had admitted to kissing the girl during a meeting with Byrne in May last year.
Sunderland suspended Johnson following his arrest on March 2 last year, but he was allowed to return to action a fortnight later and continued playing for the club until the weekend before his trial began.
The club said they had been unaware that Johnson intended to plead guilty to two of the charges facing him, but children's charities said that he should have been suspended anyway.
In a statement, the Sunderland board said that it had accepted Byrne's resignation.
"Sunderland AFC acknowledges that Margaret's intentions have always been to act in the best interests of the club," it said.
"However it has become clear through our own internal investigations that in this instance decisions have been taken by Margaret in error."
Johnson, capped 12 times by England, has been told to expect a sentence of up to 10 years.