The 17-year-old swam a scintillating final 50m to touch in 1min 04.95sec and edge South Africa's Tatjana Schoenmaker (1:05.22) into second, with King having to settle for bronze in 1:05.54.
Australia's McKeown wins Olympic women's 100m backstroke gold
Australia's Kaylee McKeown set a new Olympic record to win the women's 100m backstroke gold medal Tuesday, upsetting arch-rival Regan Smith.
The 20-year-old touched in 57.47sec, fractionally outside her own world record, with Canada's Kylie Masse, the Rio bronze medallist, second in 57.72 and American Smith third in 58.05.
"My legs were definitely hurting in the last 20," said an ecstatic McKeown.
"I'm sure it would have been pretty noticeable on the TV but I trained for that and I knew that I had a really strong back end and a really good chance to be on the podium."
Masse turned first at 50m with McKeown third, but the Australian powerfully brought it home.
McKeown had a tough lead-up to the Games, with her father dying last year from brain cancer.
But she used his memory as inspiration to swim a sensational race and smash Smith's world record at the Australian trials last month.
"It's not necessarily what I've been through," she said. "Everyone has a journey of their own and it just so happens that mine's been a really tough one.
"I wouldn't have it any other way because I don't think I'd be where I am today without all that happening."
She is also targeting the 200m backstroke gold, having recently set the fourth-quickest time in history.
Britain's Tom Dean wins men's Olympic 200m freestyle gold
Britain's Tom Dean scorched to the men's Olympic 200m freestyle gold medal Tuesday, overpowering team-mate Duncan Scott.
Dean came from behind in the final 50m to touch in a new national record of 1min 44.22sec, with Scott taking silver (1:44.26) ahead of Brazil's Fernando Scheffer (1:44.66).
"I knew it was going to be a dog fight. I didn't know how people were going to swim it," said an ecstatic Dean.
"I just want to say thanks so much to everyone, my mum, family, girlfriend. I'm lost for words, it's amazing."
South Korea's Hwang Sun-woo made the early running and was under world record pace at 100m but he faded as Dean and Scott made their move in the final 50m.
"Massive credit to Tom there, that was unbelievable," said Scott. "He's a good mate out of the pool and it's great to be able to compete against him. I'm delighted with that and I'm buzzing for Deano."
Russia's Evgeny Rylov wins Olympic men's 100m backstroke gold
Evgeny Rylov led a Russian one-two to win the men's 100m Olympic backstroke gold medal in Tokyo on Tuesday.
The 24-year-old hit the wall in 51.98sec to edge team-mate Kliment Kolesnikov, who touched in 52.00, ahead of American defending champion Ryan Murphy in 52.19.