America's Kate Douglass triumphed in the women's 200m breaststroke while Hungarian Hubert Kos took gold in the men's 200m backstroke.
Australia's dominant women's team continued their run of relay successes with victory in the 4x200m freestyle.
The 17-year-old McIntosh came to Paris Games with high expectations after stellar performances over the past two years and she is more than matching her nation's hopes.
After winning the 400m medley and taking silver behind Ariarne Titmus in the 400m freestyle, McIntosh was favourite in an event where she has already won two world championships.
But her victory was emphatic as she produced the second-fastest time in history and the quickest in 15 years.
McIntosh hit the wall in an Olympic record 2min 03.03sec ahead of America's Regan Smith and Chinese defending champion Zhang Yufei.
Her mother, Jill, had competed in the same event in the 1984 Los Angeles Games and the teenager was quick to remember that family history.
"The fact my mom did this event at the Olympics all the way back in 1984 and for me to be doing it now, I know she's unbelievably proud of me," she told Canadian broadcaster CBC.
"It's pretty unreal. The 200m fly I would say is one of my favourite races and it has been since I was a little kid. That last 50, I was just trying to feed off the crowd as much as possible. I knew it was going to be a fight to the wall."
Simone Biles's sixth Olympic gold medal hung around her neck, and so did a twinkling little goat -- just a reminder to the gymnast hailed as the greatest of all time that she does indeed belong in the pantheon of sports greats.
'Just Simone' celebrates GOAT status
"I was like, OK, if it goes well we'll wear the goat necklace," Biles said after winning a tense all-around final for her second gold medal of the Paris Games.
"I know that people will go crazy over it, but at the end of the day it is crazy that I am in the conversation of greatest of all athletes, because I just still think I'm Simone Biles from Spring, Texas, that loves to flip."
Biles, who has pushed her sport's limits on the way to an astounding haul of 39 world and Olympic medals -- 29 of them gold -- hasn't been beaten in an all-around competition since 2013 -- when she won her first all-around world title.
She won four gold medals at the 2016 Rio Olympics and looked poised to burnish her Olympic legacy in Tokyo three years ago before she withdrew from most of her events as she was struck by the mental block gymnasts call the "twisties".
"It's been eight years," she said of the gap between her Olympic all-around golds. "It feels amazing. I was a little bit naive in the process. So I appreciate my craft a little bit more."
Biles, who said she wasn't sure in the immediate aftermath of Tokyo if she would return to the world stage, credits coaches Cecile and Laurent Landi, her family and her own willingness to diligently work through mental health issues, with her ability to return from a near two-year absence and be even better than ever.
She needed all of her mental strength after a miscue on uneven bars left her in third place midway through the final, albeit just .267 points behind leader and eventual silver medallist Rebeca Andrade of Brazil.
"I was a little bit disappointed in my performance on bars," Biles said. "That's not usually how I swing.
"I'm not the best bar swinger. I'm not like Suni (Lee) or Kaylia (Nemour), but, like, I can swing some bars, you know?"
After a few minutes to "recenter and refocus" Biles delivered a solid balance beam routine to regain the lead, sealing the win with another dazzling, high-flying floor routine.
"I just couldn't believe that I did it," Biles said, adding that she was looking forward to three more finals -- vault, beam and floor exercise.
"Now it's time to have fun and the hard part is over," she laughed.
And just in case she needs it, she said, in her room at the athletes' village she has a toy goat "just to get a reminder like 'You can go out there, you can do it. You've done it before, so let's go.'"
Team GB bids farewell to Murray
The headlines for Team GB were on the tennis court as Andy Murray ended his professional career after a men’s doubles loss with partner Dan Evans 6-2, 6-4 by American pair Taylor Fritz and Tommy Paul.
Elsewhere, Team GB's rowers bagged three medals—Helen Glover and the women's four won silver, while the men's four and women's double sculls won bronze.
GB secured a spot in the quarter-finals of the Olympic women's hockey competition with a 5-2 win over the USA, while the men came from behind again to beat France 2-1 and seal an Olympic quarter-final place.
The GB trio of Ben Maher, Harry Charles and Scott Brash qualified safely for Friday’s jumping team final.
And aside from Sir Murray, there was heartbreak for Great Britain's former Olympic champion Joe Clarke, who missed out on a medal in the men's kayak single (K1) final.
British 800m medal hope Jake Wightman has been forced to withdraw from the Olympics because of a hamstring injury, denting Team GB's hopes on the track.
But in slightly more promising news, in the bowl, Team GB skateboarder Sky Brown has been cleared to compete in the Olympics after suffering a dislocated shoulder just a week before the event.
Smith bows out
South Africa's Tatjana Smith fell just short of a repeat gold in her final race at the Olympics before her retirement, being pipped by the impressive Douglass in the women's 200m breaststroke.
Smith established an early lead but Douglass was impressive at the turns as she finished in a time of 2:19.24.
The 22-year-old Douglass was the silver medallist in the 200m breaststroke at the 2023 and 2024 world championships.
"I'm really excited. For a while I wasn't sure if Olympic champion was going to be possible for me to say, and now it's just really exciting to see it happen," she said.
Smith, who won gold in the 200m breaststroke in Tokyo under her maiden name Schoenmaker and gold in 100m breaststroke earlier this week, confirmed she was bowing out of the sport.
"I think it's really just now embracing life and seeing what's outside of swimming like my passions outside of swimming," said the 27-year-old.
Hungary has a rich swimming tradition but was without a gold in Paris until Kos produced a turbo-charged final lap to overtake Greece's Apostolos Christou.
The 2023 world champion, who is trained by Michael Phelps's former coach Bob Bowman, produced a devastating final 25 metres to leave the Greek in his wake and finish a comfortable winner in the end.
It continues an impressive Olympics for Bowman's swimmers, with French favourite Leon Marchand having already claimed three gold medals.
"Without him I'd probably be like 15th in the 2IM (200m Individual Medley) right now. It's been an incredible journey with him, and I'm just to happy be part of a team like that," said Kos, who swims at Arizona State University.
"The magic touch is the work. He doesn't let us be second best. He doesn't let us stoop down to a level he doesn't want from us. That brings out the best in us."
The USA has dominated the women's 4x200m freestyle relay but Australia came out on top in an Olympic record time of 7:38.08, with Ariarne Titmus sealing the deal.
