Catlin fired a six under par final round of 64 to surge past Rai, who had led from the first round at the Galgorm Spa & Golf Resort, in Ballymena.
Rai almost foced a play-off when he birdied the 17th to put himself within a shot of Catlin. However, needing another birdie on the last hole to force a play-off, Rai found some very heavy rough short of the green.
The 25-year-old, from Perton, could only hack his ball a few feet and eventually carded a bogey to hand victory to the American – Catlin’s second of the month after winning the Andalucia Masters, in Valderrama, Spain, three weeks ago.
Rai, whose first European Tour victory came in the Hong Kong Open in December 2018, admitted afterwards: “I’m feeling a little bit disappointed. I can’t hide that.
“But I played solid for most of the week and 18 was just one of those things. It’s golf, and it happens, but John played incredibly well today and deserves it.
“It’s nice this week, it’s the first time I’ve been in contention throughout the whole week and stayed there.
“There’s a lot of positives to take, I felt like I handled myself well on the outside and, more importantly on the inside, a lot more positives than negatives to take out of this week.”
The former Wolverhampton Grammar School pupil added: “I didn’t really look (at the leaderboard) around 14 and 15.
“I was aware coming off 17 what the situation was, but that was it all day. I wasn’t trying to focus on the leaderboard. I was quite aware of what was happening then.”
Winner Catlin, who was removed from the English Open for breaching Covid019 rules in the summer added: “It’s pretty hard to describe.
“So many years of hard work have gone into this moment.
“It was my goal to win again at the start of this week, to have accomplished it is something truly, truly special.
“I had nothing to lose. I was going after every flag, I had good numbers and the greens were soft.
“So I started taking aim and hit a couple in their close. I made a long one on 15 and that three wood on 18 was so satisfying.
“It was 268 yards to the hole, cold and damp. To just hit a big high draw in there like I know I can, under those conditions, really is hard to put into words.
“(Winning at Valderrama) was massive.
“You never know if you’re going to win or not, to get that monkey off my back at Valderrama really freed me up today to know I could do it.
“Be able to look myself in the mirror and tell myself that I’ve been here before and I can do it again.
“I’ve always wanted to play in the Majors, that’s the only level of golf I haven’t played at. Hopefully one day I can win one of those as well. To have gotten inside that top 100 is a big leap forward.”
Catlin’s victory earned him around £180,000 while Rai pockets a cheque for £115,000 for his second place.
Staffordshire’s Robert Rock collected £11,000 for his 25th place finish. The 43-year-old from Armitage, near Rugeley, finished one over par for the tournament.