It was Mason's 12th goal of the season to become joint highest scorer with Peter Whittingham.
"His finish was sublime," said Mackay He feels this is the place where he will progress. Joe has worked very hard with the staff and the other players to get better all the time."
Mason lifted the ball over Leeds goalkeeper Andy Lonergan after Peter Whittingham had fired a 50-yard pass from inside his own half over the head of Leeds centre-back Alex Bruce.
Mackay added: "We haven't been able to hold him back. He's made so much progress that we couldn't leave him out of the starting XI for most of the season. That is good testament to the lad himself.
"It's about looking at talent, looking at potential and taking chances and risks on youngsters, but also doing your homework on them and making sure you believe in them.
"We have three or four youngsters like that and they will all be good players for Cardiff in the future."