Mr Mackay, who resigned as finance secretary in 2020 over messages sent to a 16-year-old boy, said there was a “high level of confidence” in the Ferguson Marine shipyard in Port Glasgow to build the vessels.
Two ferries, the Glen Sannox and the as-yet-unnamed hull 802, are now at least £150 million over budget and five years late.
A report by Audit Scotland previously found there was “insufficient documentary evidence” to explain why the contract was given to the Ferguson shipyard, which has since been nationalised, without a full refund guarantee.
Mr Mackay, who was transport minister at the time, was asked what assessment he had undertaken before recommending approval of the contract in 2015, “despite significant risks and concerns raised by CMAL,” the Scottish Government’s ferries procurement agency.
In a written response to Holyrood’s public audit committee, he said: “The submission had followed the necessary process, procurement assessment and milestone stages, therefore I had confidence in the recommendation, but appreciated that risks had been identified and understood to be resolved.
“Ongoing CMAL concerns in the event of failure were about the risks ‘to the company’ ie CMAL, and therefore that Scottish Government should give further reassurance on risk transfer to CMAL, which is covered in detail in the submission.
“Risk analysis would be expected in such a submission, with mitigations also presented.
“There was a high level of confidence in the yard, which had successfully completed other vessels for the CalMac fleet.
"There was also an expectation that there would be sufficient monitoring and oversight.
“The committee will also note that CMAL would have robustly defended a legal challenge to an award.”
Asked whether any documentary evidence exists to explain the decision to proceed with the contract, Mr Mackay said he was not aware of “any such documents”.
There has been speculation the former MSP will be called to appear before the committee in person after summer.
A Scottish Parliament spokeswoman said: “The committee will consider the content of the letters, and next steps, when it meets on Thursday.”