The migrants -- the youngest of whom were two 15-year-old boys -- suffocated in the container as they were being transported to what they had hoped would be new lives in Britain.
Haulage boss Caolan Gormley, 26, "succumbed to temptation and greed" when he became involved in the "extremely lucrative business" of smuggling migrants, judge Richard Marks said at the Old Bailey court in London.
"But for those deaths, I have no doubt whatsoever that this illegal importation of illegal immigrants would have continued, as would your involvement," he told him.
The victims, many of whom came from Nghe An and Ha Tinh provinces in north-central Vietnam, were found inside the sealed unit at a port near London on Oct. 23, 2019.
Eleven people have now been convicted over the plot in the U.K., five of them for manslaughter. Prosecutions have also taken place in France and Belgium.
A jury deliberated for just over an hour on Monday to find Gormley, from County Tyrone, Northern Ireland, guilty of conspiracy to assist unlawful immigration.
During the trial the prosecution told jurors that people smugglers exploited the victims' desperation to get to the U.K., charging more than £10,000 (US$12,600) a head.