A five-time Open winner on the regular tour to go with his two Masters titles and one U.S. Open, Watson survived a double bogey six at the last hole to capture his fifth senior major with an even par total of 284.
Watson also won this tournament at Turnberry in 2003 and Royal Aberdeen two years ago, and made it five American victories in a row after triumphs by Pete Oakley in 2004 and Loren Roberts last year. Seven of Watson's eight major triumphs in Britain have been on Scottish links courses.
Watson broke away from Ginn after a two-hole swing at the 11th. Watson rolled in a 20-foot birdie putt at a time when the Australian, who failed to make the green in two, was on a five-hole run of bogeys. Ginn ended up with a 4-over 75 for a total of 285, tied with O'Meara who finished with a 72.
Watson made up for his back-nine collapse at the U.S. Senior Open at Whistling Straits three weeks ago, when he blew a three-shot lead after 10 holes of the final round to finish fourth behind Brad Bryant.
Playing his debut tournament as a senior, Nick Faldo finished with a share of 14th at Muirfield having tied for the lead after the opening round. A final round 75 gave him an 8-over total of 292.