The Herald reported last month that Ms Mackay was concerned a discussion and decision was made in private at a council meeting in June.
Five years ago, the council agreed to underwrite any loss of profits Betfred incurred during a property deal.
The MSP said the council also paid £140,000 to refurbish new premises for Betfred in a former housing office.
Ms Mackay said: “I regretted having to issue the FOI and regret not having received any response from the council within 20 days.
“The Legal Officer, Karen Donnelly, knows only too well her obligation regarding FOIs but has chosen to ignore it, so I had no choice.
“The issue concerning Betfred is a matter of public interest as it concerns a sum of public money at a time when council workers’ terms and conditions are being eroded.
“There is a distinct lack of transparency within the mindset of this council which is simply unacceptable.
“The matter is now in the hands of the Information Commissioner and I await the outcome of that with interest.”