The woman, who spoke to the Sunday Mail newspaper anonymously, said the behaviour had a “devastating impact” on her and she had decided to speak publicly after being angered by his “media campaign”.
Mr McDonald quit the SNP and apologised last week after an internal investigation into allegations made by three women identified “persistent” behaviour including inappropriate and unwanted text messages, unwanted attention and exploiting his position of power.
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The married father-of-two, who resigned as childcare minister when the allegations first came to light in November, plans to return to the Scottish Parliament this week as an independent MSP for Aberdeen Donside.
In an interview with Holyrood magazine, he said he would stay on “to demonstrate that I have learned from my mistakes”.
But the Sunday Mail reported that the pressure of dealing with Mr McDonald’s behaviour had led to the woman ending up in hospital and off work.
She said she had chosen to stay silent during the investigation, partly out of respect to Mr McDonald’s family, but had decided that “enough is enough”.
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The woman said: “I now feel his constant media spin, attempts to gain sympathy and retain his salary have pushed me to the point where I now want to make my side public.
“I have to make it clear that his behaviour was beyond the pale and had a devastating impact.
“This was more than a few improper texts but a catalogue of attempts to abuse his position of power.
“Mr McDonald claims to be sorry but I see his attempt to return to their place of work as a blatant affront to that apology.”
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Calling for his resignation, she said she was speaking out “not only for me but for future generations of women who would work at the Scottish Parliament”.
Defending the decision in the Holyrood interview, Mr McDonald said: “I don’t dispute that I have behaved in a way that fell below the professional standards that should be expected of me but if we are to say that people cannot make a mistake and then rehabilitate and return, what message are we sending out more widely?”
He added: “I want to come back and demonstrate yes to colleagues, yes to constituents, but also to my kids that this wasn’t all that I was. That this wasn’t the sum of me, because otherwise this will be their truth.
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“It would be foolish for me to think that people are going to welcome me back with open arms... welcome me back into the fold or anything like that.
“I know I’ve got to work hard at this and prove myself all over again but I’m determined to do that.”