Thornbury came into the final run in seventh spot, but a mistake at the start saw him finish with the worst time in the fourth and final run.
The 28-year-old lost control of his sled as he started his run, with the blades coming out of the starting grooves, and while he eventually corralled it, he had lost all of his momentum, significantly slowing his eventual run down the course.
He finished with a time of 52.14 seconds, by far his worst of his four runs, and 0.5 seconds worse than the next-slowest time.
As a result, he dropped from seventh overall to 14th overall, and cut a dejected figure after the conclusion of his race.
South Korea's Sungbin Yun cruised to gold on his home course, winning by 1.63 seconds over Russia's Nikita Tregubov, who beat out Great Britain's Dominic Parsons for silver by a remarkable 0.02 seconds. In a supremely tight battle, Parsons just edged Latvia's Martins Dukurs for bronze, by 0.11 seconds.
Elsewhere, 16-year-old Queenstown ski racer Alice Robinson has missed out on the final in the slalom event, after failing to finish in her first run.
