Ronan Hughes became the second man to plead guilty over a case that shocked Britain and Vietnam and shone a light on the illicit human smuggling trade. The other is Maurice Robinson, from the British province of Northern Ireland, who was driving the truck when the bodies were found.
Appearing at a full Old Bailey courtroom in London, Hughes stood in the dock facing the judge as the names of the 31 men and boys and eight women were read out. He admitted each charge in a process that took more than 10 minutes.
Hughes also admitted conspiring to assist unlawful immigration.
Most of those who died, aged between 15 and 44, were from Nghe An and Ha Tinh Provinces in north-central Vietnam.
They were discovered last October in a container at the back of a truck on an industrial estate in Grays in Essex, about 20 miles (30 km) east of London.
A third man, Eamonn Harrison, 23, also from Northern Ireland, pleaded not guilty to the same charges.
A trial is due to begin on October 5.