ST OMER, June 16, 2013 (AFP) - The 36-year-old defeated Tjaart Van der Walt of South Africa in a play-off after both men had finished level on 279 for the tournament.
"It's so big for me," said a tearful Thornton. "On Father's Day too, he passed away three years ago and it still hurts, but they are happy tears today. This is dedicated to him.
"I didn't play particularly well today in perfect conditions but I got up and down when I had to and holed good putts.
"I didn't put myself in position very well so I was scrambling but my short game held up. I've been working hard on my short game for a couple of years now so I'm delighted. I changed my putter and my putting technique last week too so it all came together this week."
Van der Walt was disappointed not to have claimed a first European Tour victory having shared the overnight lead after every round of the week.
"It's really disappointing to get so close and not finish it off," said the 39-year-old.
Leading fourth round scores at the St Omer Open on Sunday (GBR & IRL unless stated, par 71):
279 - Simon Thornton 74 70 65 70 (Simon Thornton won in the first play-off hole), Tjaart Van Der Walt (RSA 67 71 71 70
280 - Seve Benson 75 65 70 70
282 - Robert Dinwiddie 72 66 74 70
283 - Pelle Edberg (SWE) 74 69 69 71, Chris Lloyd 73 68 70 72
284 - Jeppe Huldahl (DEN) 73 67 73 71, Agustin Domingo (ESP) 74 71 68 71, Victor Riu (FRA) 68 71 72 73
285 - Jamie Elson 75 69 68 73, Brooks Koepka (USA) 75 70 68 72, Gary Orr 73 71 74 67, Daniel Gaunt (AUS) 70 70 75 70, Max Glauert (GER) 77 65 69 74
286 - Gareth Shaw 76 69 70 71, Mikael Lundberg (SWE) 73 71 71 71, Daniel Vancsik (ARG) 73 67 75 71, Kim Sihwan (NOR) 73 71 73 69, Baptiste Chapellan (FRA) 72 68 69 77, Scott Henry 72 69 74 71, Francois Calmels (FRA) 73 71 71 71
Selected
287 - Justin Walters (RSA) 71 69 75 72
294 - Ake Nilsson (RSA) 71 74 73 76
296 - Terry Pilkadaris (AUS) 71 72 76 77