McCarthy posted two double bogeys, a bogey and two birdies on the inward nine holes at the Green Eagle Golf course to sign for an eventual 75 and T19 finish, 15 spots lower than where he started the day.
But while he will be disappointed to have let things slip late in the tournament the Challenge Tour regular will take a lot of positives from the last two weeks.
McCarthy played his first major tournament at last week’s Open Championship at Royal Birkdale after surviving final qualifying and to back up with his best finish on a major Tour suggest his game is in good shape.
Exciting English rookie Jordan Smith won the tournament in a thrilling playoff over defending champion Alexander Levy.
The pair finished regulation deadlocked at 13-under the card, Smith after a final round 71 while Levy closed with a 69.
It took two extra holes to separate the pair, Levy missing a short birdie chance at the first extra hole and Smith making him pay the ultimate price second time around.
It is Smith’s maiden victory but the 24-year-old appears to have a big future in the game having already been a standout at every level along the way.
He gained access to the secondary Challenge Tour in 2016 by winning the Order of Merit on the feeder EuroPro Tour in 2015 then graduated to the main circuit this year after winning the Challenge Tour money list in 2016.
"I've loved it. It's been nerve-wracking at times but I've really enjoyed it,” Smith said after his win."It's all a bit surreal. Obviously the Challenge Tour last year and the EuroPro the year before that. It's been fun.
"I had a really good start to the year and the first aim was just to secure my Tour card, which I managed to do."And then the second goal was to win an event, which I've managed to do."
Levy, who also won the event in a two hole playoff last year, was disappointed not to have won but pleased with the direction of his game.
"I’m really proud of what I did today. I shot three under in the conditions that were really tough and I played really well. I think there are a lot of good things coming for me in the next few months,” he said.
Dimi Papadatos was the only other Australian to make the cut and finished T60 after a final round 73.