Mahan shot 65 on the par-72 Cape Kidnappers course on New Zealands North Island to tie Kim at 8 under par after the second and final round of the four-man tournament.
A par on his second visit to the 439-meter closing hole was enough to give Mahan the win ' and a check for $1.5 million ' when Kim missed his par putt from 8 feet.
Mahan started the second round at 1 under and in third place behind compatriots Brandt Snedeker at 6 under and Kim at 2 under. The first round was played Monday at the Kauri Cliffs course in Northland province.
Mahan had four birdies and an eagle between the 12th and 16th holes Tuesday to post his closing-round 65 and tie Kim, who shot 66 for a two-round total of 136.
Snedeker shot a final-round 1-under 71 to finish third, one stroke back. Australian Adam Scott was fourth after a closing 75.