Mickelson closed with a 2-under-par 71 for a 2-over 286 total, leaving him in 18th place. He has yet to win this year and has slipped from second to 10th in the world rankings.
Randhawa, the top money winner on last year's Asian PGA Tour, finished at 8-under 276 and became the first Indian to win on the Japanese tour.
He started the final round with a two-stroke lead over Australia's Paul Sheehan. Sheehan drew even on the fifth hole but Randhawa birdied the eighth and held on to finish two strokes ahead of the Australian.
Randhawa earned $170,000.
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