The former Caley Thistle and Wigan striker netted his second hat-trick of the season as Ross County made it five wins in a row.
“If anyone asks can Billy find the back of the net, we all know the answer,” County co-manager Stuart Kettlewell said.
County went ahead after nine minutes with ex-Celt Jamie Lindsay’s through pass controlled by Mckay before he curled a shot around keeper Ryan Scully.
The hosts grabbed a crucial second shortly before the interval, with Don Cowie’s cross from the right tucked away close-in by Mckay.
Mckay completed his hat-trick within three minutes of the re-start, pouncing after Josh Mullin’s attempt was flapped at by Scully.
Individual brilliance from Davis Keillor-Dunn produced the fourth, while Brian Graham’s penalty after a trip on Declan McManus resulted in a late fifth.
“We didn’t play well,” conceded Morton manager Jonatan Johansson.