With only his second novel, the Stornoway author announced his arrival as one of the country's big hitters in crime writing.
Mackay, who lives at home with his parents on Lewis, picked up the prize at this year's Bloody Scotland book festival in Stirling for his novel How A Gunman Says Goodbye.
Based on hitman Calum Maclean, it is the second instalment of his crime trilogy set in Glasgow, a city which the 31-year-old has visited only a handful of times.
Mackay's novel pipped entries from Val McDermid, Ian Rankin, Gordon Ferris, Denise Mina, and Ann Cleeves. After receiving his award, Mackay said: "It's a huge and unexpected honour to win the Deanston Scottish Crime Book of the Year.
"At this stage of my career, I still feel like a kid being allowed to sit at the grown-ups' table when put beside talent like my fellow nominees, so just being nominated was a great thrill for me.
"The quality of the six shortlisted novels highlights the depth and variety of Scottish crime writing, and the Deanston Award and Bloody Scotland festival show what a vibrant scene it is."