The Chilean had led throughout the final day and was an inch from sinking a birdie on the 17th, before a wayward drive left him in trouble on the final hole and he unravalled as the pressure built.
An eventual double-bogey left him on four-under, putting Zalatoris and Thomas into the playoff on five-under apiece.
Thomas had started the day seven off the lead, but triumphed in the playoff to claim the The Wanamaker Trophy.
Both birdied the first playoff hole, but while a great tee shot left Thomas able to birdie the second too, Zalatoris could only make par.
And a par on the third was enough to seal victory for Thomas, with Zalatoris unable to sink a long-range birdie.
More to follow…