The six men and six women jurors began deliberations at 2 p.m. Tuesday at the Dirksen U.S. Courthouse in Chicago. Deliberations lasted approximately 45 minutes.
Trudeau was taken into custody right after the verdict after the judge deemed him a flight risk. He now faces the possibility of years in prison.
"We're disappointed in the verdict. We had hoped it would come out the other way," said Thomas Kirsch, Trudeau's attorney.
There were hugs and tears from the crowd of supporters who attended his trial daily.
"There's thousands of people around the world whose lives are changing because of him, whether he's locked up or not. And I think that speaks volumes," said Jumal Lewis, Trudeau supporter.
Attorneys delivered final arguments Tuesday in Trudeau's criminal contempt trial, with prosecutors telling jurors in Chicago that the TV pitchman intentionally lied in widely broadcast infomercials to boost sales of his diet book.
"He lied about what was in his book," said prosecutor Marc Krickbaum in his closing argument. "They weren't little fibs. They were blatant misrepresentations."
The judge will have wide discretion in sentencing with no minimum or maximum punishment stipulated in the sentencing guidelines. At the same time, Trudeau owes more than 37 million dollars from a civil judgement. The court is still trying to collect.
Trudeau, 50, could face years in prison for a conviction of violating a judge's 2004 order barring him from making false claims about his best-selling book, "The Weight Loss Cure They Don't Want You to Know About."
Trudeau, who lives in Oak Brook, a west Chicago suburb, looked on calmly as a government attorney accused him of lying repeatedly in his infomercials, which were ubiquitous on late-night television.
"The defendant lied about what was in his book, he deceived people ... for profit," prosecutor Marc Krickbaum said.
The core dispute revolves around dramatic claims in Trudeau's TV infomercials about the book, including that people who followed the diet the book could eventually eat anything they wanted without ever gaining weight.
Krickbaum said Trudeau knew such claims directly contradicted his book, the prosecutor citing paragraphs where Trudeau writes that people would have to restrict their food intake for the rest of their lives.
"He made the book sound way better than it actually was," Krickbaum said. "If he told the truth, that book wasn't going to sell nearly as well than if he lied."
The book describes a diet of 500 calories a day and the use of prescription hormones. The truth of the claims in the book itself weren't at issue in the trial - only the claims Trudeau made on TV as he sought to get viewers to purchase the book.
In his closing, defense lawyer Thomas Kirsch held up two fingers to indicate the number of witnesses the government called during the trial - a diet specialist and a postal inspector.
"They really didn't call anyone of any substance in this case," he said.
Kirsch argued that prosecutors had fallen short of their burden to prove Trudeau's guilt. Kirsch said prosecutors had also failed to show any possible misstatements in the infomercial were intentional.
In a related civil case, Trudeau has already been ordered to pay a $37 million judgment. The federal judge in that case said he was not convinced by Trudeau's insistence that he was broke and couldn't pay. Federal officials said they believe Trudeau is hiding money in secret foreign bank accounts.
The Associated Press contributed to this report.