The Venetia Williams-trained nine-year-old moved into the race soon after turning for home and began to stretch away from his rivals after jumping the final obstacle.
The outsider was well clear approaching the Elbow and passed the famous winning post 12 lengths in front of last year's victor Comply Or Die (14-1).
Paul Nicholls' My Will (8-1) was third while former Hennessy Gold Cup winner State Of Play (14-1) was fourth.
Williams said: "How can you ever expect that in a race like this?
"It was just unbelievable, the owner was watching the wrong horse for the first part of the race and she thought it was out the back.
"You just dream to have one winner when you start (training) and when you have one you dream to get to double figures, you don't dream here, this is beyond dreaming.
"I don't know what the difference has been between this year and last (when he finished 10th), if I knew I'd do it with all of them.
"He put up a career-best in the autumn when he won at Cheltenham but he hadn't run so well on his last few runs and this is such a unique race that anything could happen.
"I'm so proud of the horse. I'm so proud of Liam for giving him such a good ride, and the girl who looks after him as well.
"I'm so proud of everyone in the yard - I never get chance to praise them and give them the credit they deserve, it is all a team effort."
Treadwell added: "It's an absolutely unbelievable finish, I had the perfect run through the race, he jumped brilliant for me.
"It hasn't sunk in yet, all I can say at this moment in time is it is unbelievable.
"A couple of times loose horses fell upsides me and went under his legs but it didn't really affect the horse.
"He gave me such a great ride. He was an absolute pleasure to ride. He is so genuine.
"I had a quick look round but I wasn't really sure how far in front I was."
There were still plenty in with chances with two to jump but it was Mon Mome who went to the front after jumping the final fence under 23-year-old Treadwell.
Comply Or Die ran a quite brilliant race to fill the runner-up spot under 11st 6lb, while ante-post favourite My Will was far from disgraced back in third.
State Of Play made a number of mistakes but plugged on well to get fourth.
Tony McCoy's mount Butler's Cabin was sent off 7-1 favourite to give the 13-times champion jockey his first National success at the 14th attempt, and although he got round to finish seventh, he never really got involved in a race which remarkably saw two false starts.
David Johnson, owner of the gallant runner-up, said: "The weight has caught us out in the end but we are delighted to be second and maybe he'll go down as one of the all-time great horses of Aintree.
"He's a bit exhausted after the race so he'll have to have a few buckets of water to cool him down."
His jockey Timmy Murphy said: "He ran an absolute cracker, he jumped fantastic again.
"He probably didn't travel as well as he did last year but he had another stone on his back so you wouldn't expect him to. Personally I couldn't have been happier with him."
Andy Stewart, owner of My Will, said: "It's great for the sport to have a 100-1 winner and I'm delighted for Venetia.
"Ruby (Walsh) thought he was going terribly well two out but the others went by him. He'll probably have a bit of a break and I'd like to come back next year."
Walsh said: "He's run a cracker but he made some mistakes and when he got in deep he was hitting the fence halfway up so I had to keep kicking him and firing him into his fences, which is not ideal in a four and a half mile race, but it was the only way he would stay on his feet."
State Of Play finished very tired and was led off the track by trainer Evan Williams, who said: "He ran a great race didn't he?
"At two out I thought he was coming to win it."
Paul Moloney, rider of State of Play, said : "There was a moment when I thought I was going to win going to two out.
"Someone said the other day you can't do it without the horse and I had a lot of confidence in him.
"Early in race the only thing that was going to worry me was something falling in front of me I had so much confidence in him, and I was keen to get a good position.
"After a circuit everything had gone to plan. He made two little mistakes before Becher's second time which shuffled me back and I had to revert to Plan B but he got a second wind."
Fergus Wilson, owner of 100-1 shot Cerium who finished fifth, said: "I'm always realistic, none of my horses are Gold Cup horses but I always take my boots to Wembley.
"We're really happy and at the price our horse was I hope punters backed him to be in the first five."
The race was tinged with sadness, however, as Irish raider Hear The Echo collapsed and died on the run-in.Reuse content