The American completed the 1.6-mile Olympia course at the World Cup finals in 1 minute, 42.49 seconds after posting the fastest splits all the way down.
"It's amazing," Vonn said. "It's been a long season and it's been really difficult and challenging, but I'm really happy that it's over."
She needed at least an eighth-place finish to win the title again.
"Winning the overall a second time it's huge," Vonn said. "No American female has ever done that so it means a lot."
Maria Riesch of Germany was second in 1:42.89 and also finished second overall.
Austrian veteran Renate Goetschl, a five-time winner at Are, placed third in 1:44.39.
With three races left at the finals, Vonn has an insurmountable lead in the overall standings with 1,688 points. Riesch is second with 1,299 and Anja Paerson of Sweden third with 990.
A double gold medalist a last month's world championships, Vonn also has a chance to win the super-G title, trailing Fabienne Suter of Switzerland by only 15 points. The last super-G is Thursday.
Vonn had already clinched the downhill title for the second straight year, matching hero and friend Picabo Street's consecutive titles in 1995 and '96.
Vonn topped the final downhill standings with 502 points. She won two downhills this season and earned points in the remaining five races.
Andrea Fischbacher of Austria and Riesch were, respectively, second and third with 326 and 292 points.
"I couldn't asked for anything more," Vonn said. "I'm really, really happy."
Paerson flopped badly as she returned for the first time to the hill where she won the downhill, super-G golds at the 2007 worlds.
She started fast, but lost pace at the bottom, winding up 16th of 24 finishers, 2.38 seconds behind Vonn.
"I made mistakes where I shouldn't have made them and after that it was impossible to catch up," she said.
The race was held in snowy conditions with fog at the top of the course.