The 44-year-old is behind new venture Lochan Associates Ltd, which says it will conduct “management consultancy activities".
Mr Mackay dramatically quit as finance secretary in February 2020 on the eve of the Scottish Budget in Holyrood.
It followed revelations he had bombarded a 16-year-old schoolboy with messages, including calling him "cute" and inviting him to dinner.
Once seen as a rising star tipped to be the next first minister, Mr Mackay was never seen in Holyrood again.
Despite this, he clung on as an MSP until the May 2021 election and continued to take a full salary.
He also received an automatic grant of £11,945 for “loss of ministerial office”, and a further £53,725 “resettlement grant” when he left Holyrood at last year’s election.
The Removal from Office and Recall Bill would see MSPs automatically dismissed if they failed to take part in Holyrood proceedings for more than six months without a valid reason.
This would bring the Scottish Parliament into line with councils.
The Herald reported Lochan Associates was incorporated earlier this week and lists Mr Mackay as its sole director, sole shareholder and sole “person with significant control”.
The firm is registered at an address in Paisley, in Mr Mackay’s former constituency of Renfrewshire North and West.
Companies House says the firm will conduct “management consultancy activities other than financial management”.
Scottish Conservative MSP Russell Findlay said: “After shamelessly milking every last dime from Scottish taxpayers since his downfall, Derek Mackay's legacy is likely to be a new law being brought forward by the Scottish Conservatives that will allow rogue MSPs to be sacked.
“Perhaps the supposed one-time first minister in waiting hopes to exploit his SNP connections to continue gorging on public funds.
“But all public bodies have a responsibility to adhere to the rules and not treat him any differently to others seeking to get a foot in the door."