The 31-year-old needed only a top 15 place in the final race of the season, a giant slalom in Hafjell, and made sure of that in the second leg before sole rival Emma Aicher of Germany had left the start hut for her last run.
Shiffrin was 17th after the first run but ended up 11th, for 24 points, with 22-year-old Aicher then making a mistake and dropping from third to 12th.
"It's quite emotional," said the American. "This thing sums up the whole season of work and fighting... I have to say to Emma that her skiing has been just outstanding and today it was just so cool to watch her."
Canada's Valerie Grenier won the race, a combined 0.43 seconds clear of Norway's Mina Fuerst Holtmann and Austrian Julia Scheib, who had already secured the giant slalom Crystal Globe.
The final overall points tally was 1,410 for Shiffrin, with Aicher 87 behind.
The overall title is decided by points from all four disciplines, with Aicher an all-rounder while Shiffrin has not skied downhill for two years and competed in only a few of the super-Gs.
The most successful skier in World Cup history, with 110 wins, has, however, won nine of 10 slaloms, the bedrock of her season, and taken the globe in that discipline for a record ninth time.
She was also the overall World Cup champion in 2017, 2018, 2019, 2022 and 2023.
The only other female skier to win six overall World Cup titles was Austrian Annemarie Moser-Proell, who won five in a row from 1971-75 and her sixth in 1979. On the men's side, Austrian Marcel Hirscher holds the outright record of eight.
"I'm really, really grateful to be in this position now," said three-time Olympic champion Shiffrin, whose slalom gold medal at last month's Milano Cortina Games was another highlight of her year.
"It's really a big emotion but I'm also grateful for the fight."