World number 13 Perry came from 3-0 down to win a dramatic final in Thailand, to take the trophy plus the biggest cheque of his career: £100,000.
Perry, from Chatteris in Cambridgeshire, turned professional in 1991 but has played the best snooker of his life in recent years. He made a breakthrough by winning an Asian Tour event last season, and repeated that feat in January this year with victory in the Xuzhou Open in China. He also reached the final of the Wuxi Classic last June and came agonisingly close to the title but lost 10-9 to Neil Robertson. His only other ranking final was at the 2001 European Open when he lost 9-2 to Stephen Hendry. Today, 40-year-old Perry has finally tasted glory in a full ranking event.
Welshman Williams, who was chasing his 19th ranking title and first since 2011, looked to be cruising to the title when he won the first three frames with top breaks of 64, 57 and 103. But Perry got the better of a scrappy fourth frame then took the next two with runs of 50 and 56 for 3-3.
In the decider, Perry potted a long red and made an excellent 54 before missing a red with the rest. Williams had a chance to counter, but could only make 7, and his opponent added 24 points to seal the result.
"It has been a lifelong ambition for me," said Perry. "I've been playing snooker since I was 12, and all I've ever wanted to do is win a major tournament. When I lost 10-9 in the final in Wuxi I thought that might have been my chance. So I'm delighted to do it at last.
"I've been really composed all week. I have been very calm at the table, haven't got nervous and just felt very confident. I never expected anything other than to prepare well and try my best.
"The turning point today was the fourth frame because I didn't feel comfortable until then. It was a long frame and I managed to pinch it, and that helped me settle down. After that I was able to play my game.
"I'm really good friends with Mark and to play one of my favourite players in the final - and to win - is just the icing on the cake. The fans here have been brilliant, we've had good crowds all week. And the sponsor D88 has been fantastic, all the players really appreciate their support."
Earlier today in the semi-finals, Perry beat Stuart Bingham 4-1 with a top break of 86, while Williams saw off Judd Trump 4-2 with a top run of 85.
Both players now head to Beijing for the Baic Motor China Open, which starts on Monday and is the last event before the Betfred World Championship.
Last 32: £4,000
Last 16: £7,000
Runner-up : £38,000
High break: £2,000
Ball by ball scoring and results from tournaments