Ginn, who won the Senior Players Championship on the Champions Tour five years ago for his only major, went into the final round leading by one at 3 under, but Watson wiped that out with a short birdie putt at the third. Both players birdied the fifth to move at least four ahead of the rest of the field and the final round appeared to be a race between the two as O'Meara failed to make up ground at 1 over.
While Watson's game stayed firm, Ginn's fell apart with five bogeys in a row from the turn as he slumped from 3 under to 1 over. Ginn was wayward off the tee and, while he failed to make any birdie chances during that stretch, Watson made a 20-foot putt at 11 and two more pars to put daylight between himself and the Australian.
The victory made up for Watson's collapse at the U.S. Senior Open, where he blew a three-shot lead after 10 holes of the final round, shooting a 43 on the closing nine to finish fourth behind Brad Bryant.
"It was really disappointing, the Senior Open," Watson said. "But, as is the custom in my career, when I've had some bad tournaments, I've come back very quickly.
"I guess that's part of my personality. I hate failure. And I just want to get back to even. Winning this tournament was getting back to even."
Defending champion Roberts finished in a tie for fourth after an even par 71 gave him a 4-over 288.
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 although he almost finished birdie-birdie.
"I've struggled with my putter all week so I've got to go away and work on that before I venture out competitively again," said Faldo, a six-time major winner who now spends most of his time as a golf analyst for CBS. "Muirfield's a great walk and I can leave with a smile on my face."