World No.128 Grace closed with a six-under 65 on Saturday to finish at 13 under in the 54-hole, no-cut tournament at Pumpkin Ridge Golf Club in Oregon. The 34-year-old won $US4 million ($A5.87 million).
The fledgling LIV series, fronted by CEO Greg Norman and funded by Saudi Arabia's sovereign wealth fund, aims to challenge the PGA Tour. It has lured some players, including Dustin Johnson, Brooks Koepka and Phil Mickelson, with the promise of big signing bonuses, hefty prize purses and fewer events.
Grace beat Mexico's Carlos Ortiz by two strokes.
"Played flawless golf, played really, really well when I needed to do something special and came up and managed to pull it out," Grace said.
"But just what a great day, it was amazing to come here, this new format, this new everything is amazing and everybody here is having a blast."
Ortiz, ranked No.119 in the world, shot a 69. Johnson (71) finished four back with Patrick Reed (67).
Matt Jones was the leading Australian, tied 16th on even par after three rounds of 72.
The 48-man field in Oregon competed for a $US20 million ($A29.35 million), with an additional $US5 million prize fund for a team competition.
Even the last-place finisher, young Australian Jed Morgan, earned a payday of $US120,000 ($A176,000), despite his rounds of 76 84 77 to be 21 over par.
LIV Golf also announced that English player Pat Casey has joined the series. Casey, 44, has won three times on the PGA Tour and 15 times on the European Tour, and is ranked No. 26 in the world. He has not played a tournament round since March because of injuries.
The PGA Tour has responded to the upstart tour by suspending every active member who competed in the first LIV event. Those who played in Oregon were also suspended unless they resign their tour memberships.
LIV Golf has been dogged by criticism since its inception, well before it came to tiny North Plains, about 20 miles west of downtown Portland.
The city's mayor and 10 fellow mayors from nearby communities wrote to the course's Texas-based owner weeks ago, objecting that the event did not align with community values because of Saudi Arabia's human rights abuses, including the murder of journalist Jamal Khashoggi.
The players faced tough questions before the tournament about their involvement, with most reciting pat answers and maintaining that golf can be a "force for good." Others complained about the structure and grind of the PGA Tour.
LIV Golf touts that it's "golf, but louder." In addition to the simultaneous team competition, the tournaments feature shotgun starts, interactive fan activities and hip-hop blaring on the driving range.
Crowds on Saturday were better than Thursday and Friday. LIV Golf said it was a sellout, but would not reveal the number of tickets sold.
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