Unfortunately, the Olympic 100 metres freestyle title eluded him by the length of a finger on Wednesday and now the man dubbed "the missile" will have to prepare himself for a barrage from the Australian public.
The world champion got a taste of what might come after his disappointing lead-off leg in the 4x100 metre freestyle relay on Sunday, when he swam a second off his personal best and Australia, expecting at the very least a medal, came in fourth.
Four days on and Nathan Adrian, the American who had beaten him in the lead-off of that relay, returned to haunt him for a second time and snatched away the individual gold medal by 0.01 seconds.
Australia, an Olympic swimming superpower which has spawned some of the biggest names in the sport, had been promised a first men's champion in the blue riband sprint for 44 years and expected it to be delivered.
The promises had come from Magnussen himself, who eschewed the usual caution of an athlete before a major championship and all but guaranteed he would win, perhaps even in world record time.
"Having such a successful young career I just felt pretty much bullet proof coming into this Olympics and it is very humbling," the 21-year-old told reporters before picking up his silver medal.
"So I have a lot more respect to guys like Michael Phelps who can come to the Olympics and back up under that pressure. It is a bit of a reality check.
"As my coach said during the week, it is a pretty tough time to learn you are human."
His Olympic campaign has been Magnussen's first major setback since he exploded on to the world stage with a swim of 48.29 to win gold at Shanghai world championships almost exactly a year ago.
Another blistering finish saw him set the fastest time of the year (47.10) by some margin to win the 100 at his country's Olympic trials in March despite, he later said, being ill.
"Brace yourselves" was his message to his rivals and he repeated the warning throughout the year, adding that he thought Brazilian Cesar Cielo's world mark of 46.91, set in a now banned polyurethane suit, was within his grasp.
On Wednesday, he was fifth at the turn but as a scorching finish is his strong suit that would not have concerned him too greatly.
Adrian had taken the lead from defending champion Cielo over the first 50 metres and he just about held Magnussen off, lunging to the wall in 47.52 as the Australian appeared to need to stretch on his final stroke.
"I think I got a pretty good touch," said Magnussen. "When you lose by that much you look back and think what could I have done better but I have no regrets.
"There was a lot riding on my results at this meet so to have missed out twice is pretty shattering."
Adrian said he had been quite happy to let Magnussen take the pressure of being favourite in the lead-up to the Olympics, and that talking up his chances was not his style.
"There's a lot to be said, but until you actually get into the water and do it, it all doesn't mean anything," he said.
"So why open my mouth to provoke anybody else."
Magnussen said, however, that he had no intention of changing his ways, even if he was dubbed arrogant.
"No, definitely not, it's who I am as a person," he said.
"If you are going to be sitting where we are sitting today, there's a good chance you are going to be pretty confident.
"It's an event that takes that kind of confidence.
"The whole way through this preparation, since world championships last year, I've not been afraid to back myself and say that I wanted to win because, at the end of the day, that's the truth.
"I've been honest with the media and the people of Australia and hopefully they responded to that honesty.
"We didn't quite get the result at the end of the day but I feel like I did my best job representing Australia.
"I'm going to hold my head up high now and finish off these games."
Magnussen returns to the London pool on Thursday for the 50 freestyle but knows his chance of individual gold is probably gone.
"It's pretty early in my career, so there's plenty more Olympics to come," he said. "I'll have to go away to reassess my goals, it's going to be a pretty long four years.
"It's been a tough Olympics. They say what doesn't kill you makes you stronger so hopefully I can come out of this a better swimmer but most of all a better person."
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