Sunderland chief executive Margaret Byrne has resigned with immediate effect.
In a statement, Armagh-born Byrne said she recognises that allowing Adam Johnson "to continue to represent Sunderland was a mistake".
The player was found guilty of one count of sexual activity with a child at Bradford Crown Court last week.
The 28-year-old admitted grooming a 15-year-old girl and sexual activity with the teenager, relating to kissing her in his Range Rover, but denied the two more serious charges of sexual activity with a child.
The charges arose from a meeting between Johnson and the girl in his car in Co Durham on 30 January last year.
The chief executive said she recommended to the Sunderland board that Johnson should be allowed to play on last year because he was fighting the charges and was "innocent until proven guilty".
Byrne said that Johnson's victim "has endured a terrible ordeal and for that I am truly sorry".
She added that "at no time was the failure to suspend him again intended to cause any harm or distress to her or her family".
"I recognise that, as CEO, my involvement with Mr Johnson and the decision to allow him to continue to represent the club was a serious mistake.
"I sincerely regret that this error has impacted on the victim, the club, its supporters and all those affected in such a devastating manner."
A statement on the club's website said Byrne was "accountable for the actions taken by the club in relation to Mr Johnson".
It added that: "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."