McKay netted the opener from the penalty spot and took advantage of some slack play from the visitors to put Caley Thistle 3-1 up just after the break and take his tally for the season to 18 goals.
The former Northampton striker has now netted in seven consecutive matches and has seven goals in his last three games, topping the SPL scoring charts with 15 league strikes.
A long-range effort from Owain Tudur Jones and a solo strike from Andrew Shinnie also helped move Caley Thistle above Motherwell and take the club beyond the half-century mark in goals at the halfway point of their season.
Terry Butcher's side, who remain in both cups, have now scored 53 times in all competitions.
Dundee midfielder Nicky Riley cancelled out McKay's opener and there was little in the first half but the home side were in total control after the break.
Niall McGinn scored twice as Aberdeen beat St Johnstone at a storm-lashed Pittodrie.
The Dons were profligate in the first half, with McGinn twice denied by Alan Mannus, before the Northern Irishman scored from a free-kick after 63 minutes.
McGinn's 11th of the season followed after 81 minutes as Aberdeen defied incessant horizontal rain to end a three-match winless run in the Clydesdale Bank Premier League and climb to fourth, above Saints, who are without a win in four.
Kilmarnock host Hibernian (12:45) while Hearts face Dundee United (16.30) in Sunday's matches in the SPL.