Maria Gatland stepped down from her post at Croydon Council after being exposed as the author of a "kiss and tell" book that detailed her relationships with figures in the provisionals, written under her maiden name Maria McGuire.
Her secret past emerged following cryptic comments at a public meeting from a local Save Our Schools campaigner, who referred to the cabinet member for children, young people and learners as "Councillor McGuire".
The man, Peter Latham, then apologised for his comment, but added that he was a devotee of Irish history and had been reading a book about the IRA which "you, Councillor Gatland, might have heard about as you are Irish".
Croydon Council said news of Miss Gatland's experience with the IRA came as a "complete shock". She has been suspended from the local authority's ruling Conservative group pending an investigation
"The council has been advised that Maria Gatland has resigned as cabinet member for children, young people and learners," a council spokesman said.
"This follows emerging news of her connection to the Provisional IRA – which has come as a complete shock to Croydon. Officers understand she has been urged to resign from the council."
Miss Gatland's book – To Take Arms: My Year With The IRA Provisionals – was written after she became disillusioned with the provisionals, and fled to England. It describes how she once acted as an interpreter for the movement on an arms-buying trip to the Continent.
She decided to cut herself off from the movement after 20 bombs were detonated in Belfast on July 21, 1972, killing 11 people and injuring more than a hundred, according to the book.
"Almost for the first time, I wondered about the crippled and the widowed and the lives that had been changed forever," she wrote. She is believed to have been sentenced to death by the IRA as a result.
"I have resigned. I am not prepared to say anything more at this stage," she said.