After blitzing the host-nation's best hope for a swimming gold medal with an astonishing victory in the 800 metres freestyle final, the 15-year-old climbed out of the pool and walked straight into a storm.
Ledecky was not expected to win the gold medal, but that was not the least surprise. It was the way she won it that set tongues wagging as soon as she touched the wall.
In her first international meet, she destroyed the world's best long-distance swimmers, including Britain's defending Olympic champion Rebecca Adlington.
Never in danger of losing, she was half a second away from breaking Adlington's world record after charging ahead from the start, but even that was not the root of most astonishment.
In a sport where competitors spend years trying to shave fractions of seconds off their personal best (PB) times, Ledecky has been hacking off large chunks, more than 11 seconds this month.
At the US Olympic Trials earlier this month, Ledecky carved more than five seconds off her PB to make the team for London. She reduced it by more than another five seconds on Friday to win the Olympic gold.
"(I've) just progressively set short term goals and long term goals," she said.
"I just have been dropping time progressively and just going to some big meets and having some good races and I've been able to get down to where I am here."
While her time was outstanding, her timing was awful.
Earlier this week, an American coach, not involved with the team or US Swimming, accused China's Ye Shiwen of doping when she won the 400 individual medley gold medal, saying her rapid improve, which was less dramatic than Ledecky's, was a sign she cheated.
Ye denied any wrongdoing and swimming's world governing body backed her, saying she passed all her drug tests. The Chinese said their accusers were racist and would never point the finger at their own.
Then along came Ledecky. Within moments of her win, Ledecky's Wikipedia entry had been vandalised. At her post-race conference, the inevitable inquisition began.
"It's totally false," she replied when asked if she had doped.
"I just put in a lot of hard work, this last year and that's all that it's been. It's that simple."