The Welshman compiled breaks of 75, 64, 59 and 82 to beat Scotland's Maguire in just 43 minutes and earn a first prize of £10,000.
The tournament at the World Snooker Academy in Sheffield pitched the sport's top professionals against amateur players. Surprise results in the early stages included Shaun Murphy's 4-3 defeat against Mitchell Mann and Ryan Day's 4-3 reverse against Michael Wasley, while Brazil's Igor Figuierado won two matches to reach the last 32.
But it was a strong line up in the latter stages with Ronnie O'Sullivan and Marco Fu reaching the quarter-finals as well as Williams and Maguire. Jamie Cope beat O'Sullivan before losing to Williams, while Maguire beat Fu in the semis.
"Overall I played well, I was solid," said Williams. "I had a bit of luck in the quarter-finals, I was 3-1 down to Barry Pinches and he was on a break, I thought the match was over. Then he accidentally feathered the white, so I got back in and ended up winning 4-3. In the final, Stephen missed a few balls and I made four good breaks.
"These events are excellent, I was only there for two days and I'm going home with some money and ranking points. There are 12 events and even if you only won one match in each you'd pick a lot of points up. They're also great match practice for the main ranking events."
The next Star Xing Pai PTC event runs from July 9 to 11 at the same venue.
Results (last 16 onwards):
Ronnie O'Sullivan 4-1 Matthew Stevens
Jamie Cope 4-2 Jamie Jones
Barry Pinches 4-2 Marcus Campbell
Mark Williams 4-0 Judd Trump
Marco Fu 4-2 Stephen Lee
Joe Jogia 4-1 Nigel Bond
Tom Ford 4-3 Mark Davis
Stephen Maguire 4-3 Barry Hawkins
Cope 4-0 O'Sullivan
Williams 4-3 Pinches
Fu 4-2 Jogia
Maguire 4-2 Ford
Williams 4-3 Cope
Maguire 4-1 Fu
Williams 4-0 Maguire