McNamara had landed the closing race of day two on Wednesday and repeated the feat in the final contest of day three as the Jim Culloty-trained winner prevailed at 12/1.
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The seven-year-old jostled for the lead with Buddy Bolero in the early stages of this 3m1f event for amateur jockeys.
But Spring Heeled was soon alone at the head of affairs, although a clutch of well-fancied pursuers could be spotted travelling well in behind on the second circuit.
However, one by one they began to struggle and, while Buddy Bolero departed and 7/2 market leader Indian Castle weakened, Spring Heeled continued to travel smoothly out in front.
His fellow Irish raider Cause Of Causes emerged as the one big danger approaching the last but he jumped that one very poorly under Nina Carberry and Spring Heeled took advantage to come home a length and three-quarters clear.
Nicky Henderson's top weight Roberto Goldback (16/1) finished 17 lengths back in third, with another Irish horse, Balnaslow (9/1), fourth for Willie Mullins.
Trainer Culloty, who is famous for his three Cheltenham Gold Cup wins aboard Best Mate, was enjoying his second Festival success and his first winner of any kind since August.
"That was unbelievable," he said.
"He was a bit fractious as a young horse, a bit of a worrier and, as a result of giving him time, he ended up well handicapped.
"I thought that, if things went his way, he had a right chance, but there were 24 runners so he needed luck.
"After jumping the first I couldn't have asked for anything better as he got a soft lead, and he had the right jockey on his back to give him a great ride from the front. You could call him the winner after jumping the first.
"I hope I'm here at the same time tomorrow (when Lord Windermere runs in the Gold Cup)!"
McNamara added: "We didn't really have a game plan. Jim said to jump off handy in fifth or sixth but they didn't go mad and I was happy to just lob away in front.
"From some way out I was thinking that, as long as he jumped his fences, it would take a good one to come and get him.
"I didn't know the runner-up was behind until I was just about to land and I heard something give it a clout.
"He'd run well here before and Jim just blamed the ground the last time. Without that he could have been favourite today, Jim fancied him big time.
"This is lovely. Yesterday was more of a relief as I'd been trying for so long to get one here. I'll never forget that first one but this is magic, too."
There was an element of poignancy about McNamara's victory as well, his cousin JT McNamara having been left paralysed after a fall in this race last year.