The eight-year-old brought fledgling trainer Michael Scudamore his first success in the stamina-sapping contest, and his biggest career triumph. Michael's grandfather, also Michael, was successful in the 1957 running, while his father Peter had four wins in his days as stable jockey to Martin Pipe.
For much of the way up the straight Teaforthree and Triggerman had fought it out up front, before Paul Carberry asked his mount to put the race to bed. The winner launched himself at the fence half a stride early, causing Carberry to hang on to the buckle end of the rein.
However, the mistake did little to impair his momentum and he saw off Tony McCoy on the 11-4 favourite by half a length. Another 11 lengths back was Triggerman, with Arbor Supreme fourth.
Dual Welsh National second Giles Cross was headed four out and apparently pulled up sharply before the third-last, but trainer Victor Dartnall later reported him to be fine.
Carberry, only booked on Thursday for the ride, said: "He's a bit lairy, and I didn't want to hit the front too soon, but he kept going to the line. I haven't had the opportunity to win this so it was about time I took it. This is great."
Michael Scudamore, based 10 miles from the Welsh border at Bromsash, said he hoped the victory would act as a testament to he and his team's ability to produce winners, adding that there had been a family difference of opinion as to whether Monbeg Dude would compete.
He said: "There was a question of where we would go after he won at Cheltenham. My dad and all the owners wanted to come here, but I wanted to go back to Cheltenham on New Year's Day as I thought this might be too big an ask for the horse. Dad said we were going to the Welsh National, that there was only one place to go, and I'm delighted we did."
Jamie Moore was originally booked for the winner, but Carberry was the first-named rider after Moore told Scudamore that he had to ride for Gary Moore at Sandown.
Scudamore said: "We got Carberry when we heard Jamie was unavailable. The horse had endured all sorts of problems with his shins and his knees and it's been a great effort from the vets that he's done this. But hopefully this real boost will show people we can do it as long as we get the right ammunition."
"He ran well and did everything right," said Tony McCoy of his Rebecca Curtis-trained mount.
Ruby Walsh had to give up his intended ride on Michel Le Bon after falls in successive earlier races jarred an old injury, but he reportedly suffered no lasting damage. Ryan Mahon stepped in on the Paul Nicholls-trained horse, but he was pulled up in the marathon chase.