The former finance secretary, once tipped as a future first minister, suffered one of the most dramatic downfalls in the history of devolution.
He resigned in February 2020 on the eve of the Scottish Budget after it emerged he had bombarded a 16-year-old boy with messages.
Despite his vanishing act, Mr Mackay clung on as an MSP until last year's Holyrood election.
Since then, he's kept himself to himself. Until now, that is.
In a statement, Mr Mackay said he is "willing to co-operate" with a likely Holyrood probe into the CalMac ferries fiasco.
"To do so as comprehensively as possible I will seek access to the necessary papers and information that I am entitled to as a former government minister," he added.
His intervention comes after Nicola Sturgeon was accused of blaming him for giving the green light to a disastrous contract for two new ferries at Ferguson Marine shipyard.
The First Minister said it was "a matter of public record" that Mr Mackay was transport minister at the time the paperwork was signed, while stressing her Government "operates under collective responsibility".
Those two new ferries, of course, are now at the centre of a headline-grabbing public spending disaster.
There are serious questions around why the contract was signed off without certain financial safeguards in place and despite the concerns of Government ferries agency CMAL.
Mr Mackay is said to believe he has been "victimised". He was, after all, only a junior minister at the time.
"Anyone who knows anything about the way these processes work knows something of this size could never have happened without prior approval at the highest levels," a friend told The Sunday Times.
Mr Mackay may have resigned in disgrace, but he was previously known as a smart, capable politician, respected across the Holyrood chamber.
His intervention could prove difficult for SNP ministers at a time when they do not have their troubles to seek.
If, as expected, MSPs on the public audit committee decide to launch an inquiry, there is no doubt they will call on him.
And whatever happens next, one thing is certain – his evidence will be an unmissable Holyrood moment.