Peter Foster, 33, a marketing executive, stood impassively as the jury returned its verdict at Liverpool Crown Court.
The court was then told that he had been fined for a similar offence last year. Foster, an Australian, was also on bail for another alleged offence relating to a thigh-reducing cream.
Judge Richard Hamilton remanded Foster, of Barnet, north London, until 11 January for a pre-sentence report.
Before being led to the cells Foster, said: "I will accept my punishment. I will behave myself. I look forward to the day when I can look back to this as a very bad nightmare."
He had denied conspiring with the Young Slimmer of the Year Michele Deakin and others to supply slimming granules under the name Deakin Diet, contrary to trading standards.
Miss Deakin, 27, admitted in 1990 to the same charge that Foster faced. She was given a suspended six-month prison sentence and fined pounds 250.
The court was told during the four-day trial this week how Foster persuaded her to endorse his slimming product - made from guar gum which induced a "full-up" feeling - when she was named Young Slimmer of 1988 by Slimming magazine.
Although she never used the product, a contract was drawn up in which she would be paid pounds 1,000 a week for endorsing it.
But trading standards officers investigated after hearing complaints. People who sent off pounds 14.95 for two weeks' supply of the granules received nothing.Reuse content