Casey was part of a three-way tie atop the leaderboard to start the day at Yalong Bay Golf Club, but McGinley grabbed the lead in the clubhouse with a blistering 63. Casey answered with a 66 to match his Ryder Cup teammate at 22-under-par 266.
The duo returned to the 18th where both players made par to extend the extra session. They went back to the 18th for the second playoff hole but this time around McGinley was in trouble off the tee and sent his second shot over the green into a bunker.
Casey found the fairway and played his approach inside 25 feet. McGinley punched out of the trap, but Casey calmly drained the birdie try for the win.
'I felt that I played good golf all week and I think the reason for that was that I felt quite at home here,' Casey said. 'The place is fantastic and the people are so friendly, and the course also reminds me of a few back home in England.'
Casey had a solid start with a pair of birdies over his first three holes to climb to 18 under. He added a birdie at the seventh and another at the par-5 ninth.
The Englishman soon found himself trailing McGinley, but Casey persisted with a birdie at the par-4 14th to get within one of the lead. Casey then birdied the 16th and had a chance for the outright win at the closing hole after his second shot stopped within 15 feet of the cup. Casey missed the birdie putt, however, but held on in the playoff for the win.
McGinley jumped out of the gate with a birdie at the par-5 first and picked up three more birdies on the front nine to make the turn at 17 under. The Irishman then caught fire on the inward half with five consecutive birdies starting at the par-5 13th to earn a spot in the playoff.
Thomas Bjorn shot a 66 for his fourth top-5 finish of the season. Bjorn finished at 21-under-par 267 along with Chawalit Plaphol and Wook-Soon Kang.
Colin Montgomerie, who shared the third-round lead with Casey and Plaphol, managed a 4-under 68 to take sixth place alone at 20-under-par 268. Ivo Giner, Edward Loar and Wen-Teng Lin were one shot further back at 19-under-par 269.
Alex Quiroz, the leader after round two, carded a 69 to share 10th place with Wen Teh Lu at 17-under-par 271.