The 18-year-old American pipped Tina Maze to the slalom crystal globe with a blistering performance on the final section of the second run.
Both women sobbed in the finish area - Shiffrin with joy and Maze with the frustration of finishing third and missing out on the slalom title to go with her overall, giant slalom and super-G cups.
Shriffin had been 1.17 seconds behind Maze after the first leg but beat the best overall skier of the season by 0.35 seconds with her final combined time of one minute 55.60 seconds.
Worn out after her exceptional season, Maze finished 0.15 behind runner-up Bernadette Schild of Austria, the sister of former world champion Marlies, who took the first podium of her career.
"I actually want to thank Tina Maze, she really pushed me hard," said Shiffrin.
Shiffrin finished the slalom season on 688 points, 33 more than Maze, who took her overall total to 2,314.
Maze will ski her 40th and final race of the season in Sunday's giant slalom on the same Lenzerheide course, hoping to end the winter on a last victory.
- Sports & Recreation
- Tina Maze