He was universally known as "Slasher", an ironic reference to his often back-to-the-wall batting style, which was bestowed upon him by Toombul District Cricket Club teammate, Aub Carrigan.

He matured as a Test player to become an unobtrusive but an often vital member of Richie Benaud's team, that brought Australia out of its late-fifties doldrums in two remarkable series, against the West Indies in 1960–61 and England in 1961.

Mackay made important contributions in both, most notably his famous last-wicket stand with Lindsay Kline in the 4th Test against the West Indies in Adelaide which forced a remarkable draw.

Mackay teamed with Benaud and Alan Davidson to provide a high-quality, flexible core of all-rounders that often proved the difference for Australia in tight situations.

While lacking the talent of the fast left-arm swing of Davidson and the leg-spin of Benaud, his economical, nagging right-arm medium pace was often strategically useful and occasionally, especially in Pakistan and India, destructive.