"I think that the first time that Derek rode him this year at Kelso he came back and said, "I've never ridden a horse like that", and it's just great for him now to win the Grand National".
The Lucinda Russell-trained gelding enjoyed a thrilling sprint down the home stretch, having battled with Cause Of Causes over the final few fences.
One for Arthur jumped so jolly well, apparently loving every minute of his four mile journey, shaking Cause of Causes 16-1 by 4.5 lengths, with Saint Are, third at 25-1.
One For Arthur stayed on best of them all to give Scotland only its second ever victor in the Randox Health Grand National at Aintree.
"This is the best feeling I have ever had, or ever will have", Fox said.
"I was lucky to get back in time - I want to thank Jack Berry House".
Should Vieux Lion Rouge prevail it would see jockey Tom Scudamore emulate his grandfather Michael who won on Oxo in 1959 and achieve something his father, multi-times champion jockey Peter, failed to do.
It is Scotland's first victory in the event since 1979.
McIlroy needs big final round for career Grand Slam
Two birdies in the opening three holes took McIlroy to within three of the lead but his failure to capitalise on his magnificent driving cost him dearly.
All 40 horses which competed in the steeplechase returned safe, although not all finished. I can't believe it.
McClung added: "I thought this morning, it's baking hot so there's no pressure now, he's not going to win on that ground but I have to say he's just shown his class today".
Russell was becoming the fourth woman to train the National victor after Jenny Pitman, Venetia Williams and Sue Smith, and went on: "It means everything".
They have purchased Le Mercurey and Highland Lodge, who would give jockey Henry Brooke a fairytale win after a crashing fall saw him placed in an induced coma earlier this year, while their son Richard has bought Just A Par.
"He's fantastic. He's improved very time".
Ms Thomson said: "He's a superstar, dreams have come true for us really".
The victor is owned by childhood friends Belinda McClung and Deborah Thomson - who chose the name "Two Golf Widows" for their ownership partnership in a dig at their husbands when they teamed up to buy him. It's been an absolutely fantastic day. I kept thinking barring accidents, he would win the National and he has. Barry (Geraghty) said he gave him a great ride round there, but he just didn't get home.