Susan Gaszczak announced her decision in an open letter to party leader and Deputy Prime Minister Nick Clegg after an appeal against a disciplinary review - which found there was insufficient evidence to show the peer broke party rules - was dismissed.
In May, Lord Rennard finally issued the apology to four women who complained about him that had been recommended by Alistair Webster, QC, whose inquiry concluded there was broadly credible evidence the peer had violated personal space.
But Mr Clegg remains under pressure to eject the former elections supremo - who remains suspended pending his own appeal against the handling of the case - from the party altogether.
In her letter, published on social media, Ms Gaszczak said: "I can no longer remain a member of a party that feels it is acceptable for the then chief executive to invite me to his hotel room to advance my political career.
"Even though I have complained, and raised this for over seven years, the party still do not see the complaint as an issue.
"This week the appeals panel met and threw out our appeal to an unfair process.
"I have been called credible more than once. Yet still no-one feels it is appropriate to deal with the issue."
She went on: "Only four of us were brave enough to go public - how many more do you need on public record? You have a serious thorn in your side. He claims to have built the party, but actually I see him destroying it."
Ms Gaszczak said her mother and father - himself a councillor for 24 years - had followed her out, while her son and daughter had also resigned.