'I can't believe it'
Speaking to Channel Seven moments after the win, Titmus says she is as shocked by the result as anyone, despite her careful planning and preparation.
"I tried to stay as composed as I could. Then just tried to stick to my race plan. I can't believe I pulled it off."
She said training during the pandemic has had its challenges, and she is happy to have delivered the gold.
"I can't believe it. I'm trying to contain my emotions...This past year I don't know whether it's gone fast or slow, but to get here was a relief. To come here and do the job, I'm over the moon."
Her coach, Dean Boxall, could not contain his joy with the cameras capturing his live and enthusiastic reaction, fists pumping the air.
Titmus' career highlight achieved at her first Olympics
It delivers Ledecky, regarded as the greatest female swimmer ever, her first defeat in an Olympic final.
This is Titmus' first Olympics, although she has been in the Australian Swim Team since December 2016.
Titmus, in lane three, kept watch on Ledecky in lane four in what became the highly anticipated match race between the pair.
The American held the lead for the initial 300m but Titmus was watching her all the way, literally lurking at her heels.
The young Australian ominously surged closer and was just 0.16 seconds behind Ledecky with 100m remaining.
Titmus then reeled in her rival in a perfectly-executed race plan to win by half a body-length in a time of three minutes 56.69 seconds.
Ledecky touched home in 3:57.36 and China's Li Bingjie was well back in third position in 4:01.08.
Ledecky missed out on the gold by just 0.67 seconds, even though her time was the second-fastest of her career and the fourth-fastest in history.
She said she gave the race her all.
"I fought tooth and nail and just gave it my all, so I can't be disappointed by that," Ledecky told NBC after the race. "It was a good time by me, just an awesome swim by her."
Titmus and Ledecky will also square off in a much-hyped 200m freestyle battle, though the American is favoured to win their duel over 800m.
Titmus has in the past downplayed any "bad blood" or "rivalry" between the pair.
'We have this battle going'
She used her interview with Channel Seven after her win to pay tribute to her competitor.
"I thanked her, I wouldn't be here without her. She set this incredible standard," Titmus said.
"I've been trying to chase her... It's really exciting now we have this battle going. It's really fun to race."
Titmus' time in the Olympics event was 1.8 seconds faster than her personal best, setting a new Australian and Commonwealth record.
Anticipation was high ahead of the showdown between Titmus and Ledecky, after the Australian came from behind to defeat Ledecky by more than a second in the 400m freestyle at the 2019 World Championships in Gwangju.
At the Australian trials in Adelaide in June, Titmus achieved the second-fastest time ever in a Women's 400m freestyle with a time of 3:56.90. That was a hair behind Ledecky's World Record from the Rio Olympics of 3:56.46.
Another opportunity for gold medals in the pool will come with the men's 4x100m freestyle relay team event on Monday.
Australia is now ranked sixth on the medals table, behind Italy in place number five. China leads the medal tally.
Additional reporting: AAP