“What an aptly named horse,” said mud-splattered 19-year-old Mullins, who only turned professional 17 months ago.
Rule The World was third after jumping the last of the 30 fences, and outpaced The Last Samuri – the 8-1 joint favourite – and 100-1 outsider Vics Canvas in a thrilling chase to the winning post in front of about 70,000 punters at Aintree.
Rule The World achieved his first victory in 14 races over fences in arguably the biggest of them all. Winning such a gruelling race in wet conditions was all the more the remarkable given the nine-year-old horse twice previously broken its pelvis, nearly ending his career.
“When you consider the injuries he has been through, you can only call him a horse of iron,” said trainer Mouse Morris, who got the biggest win of his career less than a year after his son, Christopher, died from suspected carbon monoxide poisoning while travelling in South America. It’s like Disneyland, fairy-tale stuff. Someone is looking down on me.”
He added: “He’s fractured his pelvis twice. Before that I always thought he was the best horse I ever had, how good would he be with a proper rear end on him?
“He had a nice weight and he’s a class horse on his day. I know he was a maiden (over fences), but he’s been running good races – Grade One races – and banging on the door.
“This is next best to the Gold Cup (won with War Of Attrition in 2006).”
Mullins is the nephew of leading Irish trainer Willie Mullins, and the grandson of Paddy Mullins, who is best known for training the great mare Dawn Run to a victory in the Cheltenham Gold Cup in 1986. His father, Tom, is also a trainer.
Despite his family’s links, David Mullins never really liked horse racing and only got into it at the age of 15 because he did not know what else to do.
“I wasn’t the cleverest kid in the class, I was no good at any sport, and my dad was giving me a kick up the backside every Saturday morning telling me to go and do a bit,” he said.
He barely remembered walking the Grand National course with family members as a nine-year-old, but he won’t forget his first ride on it.
Mullins followed pre-race orders in staying on the outside of the field and keeping Rule The World out of trouble, all the more important given the heavy ground that led to 23 of the starting horses falling.
Rule The World moved into contention toward the end of the second circuit of the 4½-mile (7.2km) course and recovered from a mistake at the fourth-last. His finishing kick was too strong for Last Samuri, who had to settle for second place, six lengths back.
Mullins said: “It’s unbelievable. I just couldn’t expect things to have gone better.
“There was one little mishap at the fourth-last, but thank god I came out (the other side). Everything went to plan really.
“Credit to Mouse, he’s produced this horse without having won over fences. Then there’s me, who’s never even walked around the Grand National track (as a jockey).
“Mouse is a genius and he’s the best man in the world for preparing a horse for one day.
“I’m very thankful to Michael and Eddie O’Leary (of Gigginstown stud) for giving me the chance.
“That’s the best ride I’ve ever got off a horse and it’s the best feeling to come back into a place like this. It was just brilliant.”
Rule The World’s owner, Michael O’Leary, won the Cheltenham Gold Cup last month with Don Cossack but said he never expected to back that up with a winner of an even more globally recognised race. It was only his second entry for the Grand National.
“I don’t know what to feel, I’m numb,” said O’Leary, who is the boss of Irish budget airline Ryanair. “I thought I had no chance in it, I wanted to win a Gold Cup and it was beyond dreams that I could win a Grand National.
“To win a Gold Cup, Irish National and Grand National in one year – I think I should stop, it’s not going to get any better than this.”
Trainer Kim Bailey said of The Last Samuri: “Nothing is over until they cross the line, the horse tried his heart out and he jumped for fun.
“It’s the longest run-in you can possibly imagine. I was standing here screaming – my voice has gone.
“We’ve beaten the third horse, but another horse has come on the outside from nowhere. I’m just so proud. We’ll do it all over again next year 12lbs worse off.”
Owner Paul Rooney said: “He’s only a baby at seven years old, next year is his year. This is his trial run and we’re very confident about next year.”
None of the 39 starters was hurt, with the three-day festival having been marred by the death of four horses over the first two days.
Many Clouds, the 2015 winner, started as the joint-favourite but ended up last of the 16 finishers.