Mackay is currently Waitrose's wine buyer and joined the retailer in 2008, having become the youngest ever Master of Wine in 2006.
He will be responsible for buying Champagne and sparkling wine, as well as leading a team of two wine buyers and a buying assistant.
Pierpaolo Petrassi MW, head of buying for beers, wines, spirits and tobacco, said: "We are delighted to be able to promote Ken to this senior role. The subsequent addition of a second buying manager to the wine buying team will give us the structure to pursue the considerable opportunity for growth within this flagship category for Waitrose."
Waitrose added that it was still looking to recruit a second buying manager to replace Andrew Shaw, who leaves Waitrose at the end of September.