McCarthy - the world's 1342nd ranked golfer - was the only one of 13 Australians to play his way into the field at Royal Birkdale, those dozen players spread over five venues across England and Scotland.
The 27-year-old's two-under total of 140 saw him into a playoff against three others for the one remaining spot of the three available at Gailes Links in Scotland, which he won with an eight-foot birdie on the first extra hole.
McCarthy, one of Australia's best amateurs before turning professional, has been a professional for five years and his success today is likely one of his most satisfying moments in professional golf.
Nick Cullen agonisingly missed a playoff for one of the three spots at his venue at Royal Cinque Ports, a second round of 72 not quite enough for him to be playing yet another Open Championship.
Richard Green finished tied for 4th at his venue at Hillside Golf Club in Southport, missing out on a chance at one of the three places by two shots.