Poised to end a decade long major drought, McIlroy collapsed at the finish with bogey's on three of his final four holes, including a heart-breaker at the 18th where he missed from inside four feet.
At the same time, playing in the final pair, DeChambeau sent his tee shot into the rough but held his nerve to finish with par and a final round one-over 71.
That was good enough to earn the LIV Golf standard bearer a heart-stopping victory with a six-under 274 winning total.
McIlroy had a final round 69 to finish alone in second one ahead of American Patrick Cantlay (70) and Tony Finau (67).
The 30-year-old American started the day at the top of the leaderboard three clear of the chasing pack that included McIlroy but trailed the world number three by two with five holes to play.
McIlroy had used a red hot putter to snatch the lead, but with the finish line in sight, suddenly went cold on the greens twice missing from inside four-feet at 16 and 18 leaving the Northern Irishman with a stunned look as he walked off the course.
