Reed birdied the par-five 18th to win the opening event of 2015 after a rollercoaster finish to the tournament that included holing from the fairway for what turned out to be a vital eagle.
The Texan, like Walker, was a Ryder Cup rookie at Gleneagles last autumn, but his hopes of victory looked slim when he trailed by four shots with four holes remaining.
But, after birdieing the 15th, Reed holed out from 80 yards for a two at the next and then picked up another shot at the long closing hole after a bogey at 17.
Walker was left kicking himself after a sloppy back-nine performance at the Kapalua Resort.
He looked to be on his way to victory when he reached the turn in four-under 32 and added a fifth birdie at the 10th, but he would go one-over for the rest of the round, failing to make a chip count on the final hole as he looked to avoid the play-off.
And a par was not good enough for him at the opening extra hole as Reed rolled in his putt to secure the $1.14m cheque for first prize.
“I just stayed patient. That was the main thing all day,” Reed told ??Sports 4. “Just stay patient and try not to get ahead of myself and make any??careless errors.
“(My) confidence levels are really high, it’s been a great calendar year, but??at the same time I need to keep working at my game, tighten up some things and??continue moving forward and hopefully be a multi-winner in years to come.
“Everyone gets on a hot streak, the main thing is to stay consistent and keep??moving forward.”
Third place ended in a three-way tie although the headlines were dominated by Jason Day and a course-record equalling 62.
Chris Kirk had set the new mark earlier in the day, playing on his own after ending the third round in last place, but it was matched by the Australian who carded 11 birdies.
Russell Henley finished with a 67 to join Day and Hideki Matsuyama joined them, closing with a 70 that could have been so much better but for a series of missed putts down the stretch.