McKay, who has become a key player in Holger Osieck's Australia side in the past 12 months, helped Brisbane Roar win the 2010-11 A-League title before recently joining Scottish champions Rangers.
The 28-year-old midfielder won the award at Tuesday's Australian Football Awards ceremony in Sydney with 329 votes, ahead of Holman (194), Cahill (185) and Argentine former A-League star Marcos Flores (109).
An ecstatic McKay told AAP: "I'm still pinching myself. This time last year I got my first call-up to a full-strength Socceroos, I was excited just to meet Harry Kewell, Tim Cahill and Mark Schwarzer and now they are my mates.
"It's a huge honour for me and I want to be part of the Socceroos for a long time."
McKay paid credit to Brisbane coach Ange Postecoglou and Australia boss Osieck for his rapid rise.
"Ange and Holger have given me opportunities, and I guess it shows what a bit of confidence can do for a player," he said.
"It's been a stellar year, I have really been enjoying my football, my time with the Socceroos and now I'm hoping to continue to improve my game further."
Meanwhile, Fortuna Dusseldorf striker Robbie Kruse won the Harry Kewell Medal for the best Under-23 player, while Central Coast keeper Mat Ryan won the U-20 Footballer of the Year award.
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