Ryder Cup captain Tom Watson would have wished to have Billy Horschel in his American team after seeing him win a second straight tournament against a world-class field but the deadline to make selections has already been reached.
Tiger Woods, who missed the cut in Washington on his comeback from back surgery, stayed in fifth place behind two other Americans, Bubba Watson and Matt Kuchar.
Second-round leader Thomas Bjorn of Denmark shot 71 and was in second place, while American Matt Kuchar had 68 and was in third place, three behind.
Woods won the Jack Nicklaus Award for the 11th time in his career on the back of his five big wins and return to No. 1 in the world this year.
The World Cup of Golf is to be held from November 18-24 at The Royal Melbourne Golf Club.
Kuchar, winner of the WGC Match Play Championship at Dove Mountain, joined world number one Tiger Woods as the only multiple winners on the US PGA Tour this season.
Mahan was the first defending champion to return to the final since 2006 winner Geoff Ogilvy of Australia was runner-up in 2007.
American Matt Kuchar kept his composure while others around him caved under the pressure to win The Players Championship tournament by two shots on Sunday.
Putting machines Matt Kuchar and Gary Woodland on Sunday claimed the United States' first golf World Cup since Tiger Woods and David Duval lifted the trophy 11 years ago.