The 22-year-old, a four-time Olympic gold medallist, scored 118.5, with 16-year-old Sunisa Lee second on 113.5.
The win moves her level with Clara Schroth Lomady, who won her sixth US all-around title in 1952.
Biles became the first woman to attempt and land a triple-twisting, double back manoeuvre during her floor routine.
“It wasn’t as good as in training,” said Biles, who scored 29.45 for her floor routine.
“I’m just happy that I landed it because I feel like after night one my confidence got shot down. So I was really worried about it going into today, that’s all I could worry about. So I was really happy.”
Biles missed the championships in 2017 but has now won the US all-around title six times in seven years.
She has now won 20 straight all-around events in which she has competed, a run dating back to 2013.
Spain wrapped up their successful Euro 2020 qualifying campaign with a 5-0 win against Romania.
Fabian Ruiz gave the three-times European champions an early lead with a close-range finish before Gerard Moreno flicked home Santi Cazorla’s cross.
Moreno side-footed in his second of the game before Adrian Rus turned the ball into his own net to make it 4-0 before half-time.
Mikel Oyarzabal swept in a fifth deep in stoppage time to complete the win.
Spain topped Group F with 26 points after winning eight and drawing two of their 10 games.
Sweden, who like Spain had already secured their place at Euro 2020 before the final round of fixtures, finished as runners-up on 21 points.
|2019 Nitto ATP Finals|
|Venue: O2 Arena, London Dates: 10-17 November|
|Coverage: Watch live coverage of one match per day on BBC TV, BBC iPlayer and online; Listen on BBC Radio 5 Live Sports Extra; Live text on selected matches on the BBC Sport website and app. Click here for Live Guide.|
Sixth seed Stefanos Tsitsipas won his opening match at the ATP Finals by edging out fourth seed and fellow debutant Daniil Medvedev in London.
The Greek 21-year-old beat the Russian 7-6 (7-5) 6-4 at the O2 Arena.
There was just one break of serve in the match with Tsitsipas, the youngest player in the tournament, earning his first win over Medvedev – a player with whom he has a tense rivalry.
“It is one of the toughest and most important victories,” Tsitsipas said.
“It’s not easy coming in knowing you’ve lost five times before [to Medvedev] but this victory means a lot to me, this crowd means a lot to me.
“[There are] Greek flags everywhere – it almost feels like I’m playing in Athens.”
World number one Rafael Nadal plays defending champion Alexander Zverev later on Monday in the same group.
Novak Djokovic, who can overtake Nadal at the top of the world rankings this week, leads the other group after winning on Sunday.
The top two players in each group after the round-robin stage progress to the semi-finals on Saturday.
The men’s season-ending event, which is taking place in London for the penultimate year, features the top eight players of the year.
‘We won’t go for dinner together’ – Tsitsipas on Medvedev rivalry
Tsitsipas had lost all five of his previous meetings with US Open runner-up Medvedev and the pair have had a difficult relationship since a row over a net cord in Miami last year.
Tsitsipas then called the Russian’s playing style “boring” after a defeat at the Shanghai Masters last month.
The pair are two of the most highly-rated young players in the game – two of four singles players aged 24 or under at this year’s championships – but Tsitsipas was rewarded for being the more aggressive of the two players.
Asked whether beating Medvedev meant extra to him, the Greek said: “It means more than extra.
“It’s a victory that I’ve craved for a long time now and it’s great that it came in this moment.
“Our chemistry definitely isn’t the best that you can find on the tour. It just happens with people that it’s not that you can just like everyone.
“It’s not that I hate him. I guess, as he said, we will not go to dinner together.”
There were no breaks in the first set but Tsitsipas played better in the tie-break, earning the crucial mini-break to move 6-5 ahead by winning a gruelling point at the net with a volley.
The second set was similarly tight but Tsitsipas remained immaculate on serve – he did not face a break point in the match – and eventually broke serve to go 5-4 in front, helped by the Russian choosing not to volley a return at 30-30 that landed in.
Tsitsipas served out the match in the following game, and celebrated enthusiastically after the final point as he moved top of the early group table.
Tsitsipas, who beat Federer en route to the semi-finals of this year’s Australian Open, won the Next Gen event a year ago – the season-ending tournament for players aged 21 or under.
He said he had “goosebumps” when making his debut in London.
“I watched this event for the first time in 2010, I used to watch on TV dreaming of potentially playing on these courts,” he said.