Lin Liang Ren had taken the workers to Morecambe Bay in Lancashire to pick cockles, small edible mollusks which live in underwater sand, at low tide.
When the tide turned, the cockle pickers, many of whom could not swim, were trapped out in the bay. Twenty-three of them drowned; only 21 bodies were recovered.
Morecambe Bay, some 300km northwest of London, is said to hold millions of dollars worth of cockles, which are especially popular in Spain. However, it is also known for its quicksands and rip tides, and local fishermen have lost horses and tractors to the sands.
Lead prosecutor Tim Holroyde told the court at the trial's opening last September that the Chinese men and women, who as illegal immigrants had lived as slaves, drowned due to the "goldrush fever" for cockles.
Outside the court, Detective Superintendent Mick Gradwell described the workers' "horrendous" final minutes, trapped by the rising water in the dark. "It was bitter and they'd been working hard. Listening to some of the phone calls (they made) you can hear the harrowing noises as people tried to save themselves."
The court on Friday also found Lin and his girlfriend Zhao Xiao Qing guilty of perverting the course of justice and facilitating illegal entry into the country. Lin's cousin Lin Mu Yong was also found guilty of facilitating illegal entry.
"Listening to some of the phone calls (they made) you can hear the harrowing noises as people tried to save themselves"
The court cleared an English father and son, both named David Anthony Eden, of the same immigration charge but the judge insisted they pay their own costs.
Manslaughter carries a possible life jail term as does perverting the course of justice. Sentencing is due on Tuesday.
The case made international headlines and highlighted the plight of immigrant - and often illegal - workers, unprotected by national labour regulations.
Following the tragedy, the government moved to regulate gangs of migrant workers.
The legislation, which will apply to some 600,000 workers, will be extended to commercial shellfish gathering from October.