Iwilldoit, trained by Welshman Sam Thomas, backed up his victory in the trial to win the Welsh Grand National.
Paul Nicholls’ wait for another winner of the race continues, but his horses Highland Hunter and Truckers Lodge finished second and third.
Last year’s winner, Secret Reprieve, looked to have a strong chance with four flights left to jump but faded and finished fifth.
Achille finished fourth with only five horses out of 20 completing the course.
Backed into 13-2 from 12-1 on the morning of the race, the eight-year-old went to the front eight fences from home and ground down the opposition.
It was a memorable home triumph for Thomas who follows Christian Williams (Potters Corner, 2019) and Evan Williams (Secret Reprieve, 2020) as Welsh winners of the race at Chepstow.
Before Williams’ win two years ago, it had been 54 years since a Welsh trainer had won this race.
Thomas had previously won the 2008 Cheltenham Gold Cup as a jockey riding Denman.
“I can’t put it into words, I can’t believe how emotional I am,” said Thomas.
“I didn’t dare think for a minute that we were going to win. I know we had a chance, but it is incredible.
“I’m a bit overwhelmed by the whole experience.
“It is tremendous to keep it in Wales. I am a proud Welshman and it hasn’t sunk in. It is what dreams are made of.
“Big targets keep your eye on the ball and it is what gets you out of bed every day. I have a fantastic team at home and this is a team effort.”
It was also an impressive success for jockey Stan Sheppard in an attritional race staged without spectators due to the Welsh government’s Covid-19 regulations.
“It is very bizarre being stood here in an empty stand with no crowd, it just doesn’t feel right,” added Thomas.
“I have enjoyed every minute of that race, but my family should be here, the rest of the owners should be here, and it is a very sad day for racing.
“It just doesn’t feel right – 40 minutes away in Bristol there is a full stadium of people watching rugby. It is crazy.”
After two false starts, predominantly due to Highland Hunter’s eagerness, the race began from a starting start, with Ramses de Taillee and 2019 winner Potters Corner leading for most of the first circuit.
But with a circuit complete, the field began whittling down quickly, Discortantly was the first to fall before Ireland’s other chance, The Big Dog, made a bad mistake at the ditch and was pulled up by Jonathan Burke.
After the 15th, the tailed off group, including former winner Native River, pulled up leaving eight horses with a chance rounding the final bend.
But Iwilldoit, who took the lead with half a circuit to go, held off the challenge of Highland Hunter, and those two finished a long way clear of Truckers Lodge in third.
“When he hit the front down the back and got into his rhythm, I thought all he had to do was stay on his feet really and he’d keep running is race,” added Thomas.
“Stan gave him a lovely ride – it is a lot of pressure for a young jockey, but I’m delighted for him.
“He loved those conditions and bottom weight in those sort of races obviously helps in soft ground. Hopefully, I will be able to enjoy it a little more later.”