The pair began the evening session with the Thai leading 9-8 and needing just a solitary frame but Maguire dug deep to take it to a decider after a tense exchange on the final brow. The popular Thai looked flustered, shaking his head before the decider but he held it together to compile an excellent 63 break before running out of position.
Maguire still had a chance but when he missed a tough red to the middle, the Thai cleared up before punching the air in delight. He now plays Michael White in what will be one of the more attractive second round clashes.
"It's a big surprise because Maguire is a top-five player and I thought I only had a 10 or 20 per cent chance of beating him," said Poomjaeng.
"I believed in myself. I knew it would be very hard but I hoped I could win if I got chances. I tried to just relax and enjoy it because I knew if I was too serious it wouldn't be good for my game. It was exciting at the start because there were lots of people and it was my first time at the Crucible.
"I hope I play well in my next game, but win or lose I'll be happy. I'm happy that people like me. I enjoy the game and hope people enjoy it with me."
Maguire said: "I was afraid to look at him in the end. Crazy things were happening out there. I just had to switch off. All I can say is that he's a nice lad.
"It just wasn't meant to be for me. Take nothing away from him, he made some great breaks all through the match and luck favours the man who plays well and he played better than me.
"It's disappointing for me but I could tell the way the match was going. I had a bad feeling about it."
SECOND SESSION REPORT
Dechewat Poomjaeng held his nerve to edge 9-8 ahead of Stephen Maguire, but the slow pace of play meant their match must go on again after the conclusion of the evening session's snooker.
Poomjaeng, the world number 70, came through four qualifying rounds to reach the televised stages. He handled himself superbly well after last's year semi-finalist Maguire had recovered from trailing 5-3 overnight to lead 6-5.
They play to a finish when the first table becomes available from tonight's evening session after they failed to finish in their allocated time slot this afternoon.
Scotsman Maguire - the world number five - made 70, 91 and 93 to move ahead in the match, but a bad kick in the 12th frame allowed Poomjaeng - who has smiled constantly throughout the match - to run in 74 in levelling at 6-6.
They swapped frames until 8-8 before the Thai player contributed 54 to move one up with two frames to play as a clearly disgusted Maguire conceded the frame needing one snooker.
FIRST SESSION REPORT
Stephen Maguire struggled to find his best form as he fell 5-3 down to World Championship debutant Dechewat Poomjaeng at Sheffield's Crucible Theatre.
Thai player Poomjaeng - the world number 70 who came through four qualifying rounds to reach the televised stages - made a blistering start to the afternoon as runs of 72, 59 and 60 helped him ease into a 3-1 lead at the mid-session interval with Maguire running in 69 to win the third frame.
A 50 handed Poomjaeng - who is bidding to become the first Thai player to win a match at the Crucible in 14 years - the fourth frame, but Maguire cancelled out a 56 from his opponent with a 63 to claim the sixth frame.
The final two frames of the session were shared with former UK champion Maguire happy to snare the closing frame of the session as the players were hauled off a frame before their allocated nine for the session were due to be completed.
They play to a finish on Tuesday afternoon with the first man to 10 securing a meeting with Michael White in the last 16.