He was remanded in custody to be sentenced at a later date.
The court heard the former cab driver's "KF Concept" roadshow deliberately targeted low income people, including many in small valleys communities in south Wales.
At least 8,000 people "invested" a minimum of £1,000 into the scam, with a guarantee that within two years they would get a return of five times their initial outlay.
During the roadshows he appeared on stage to music, cheering and handing prize winners luxury holidays and even free cars.
The jury was told he used their money to fund a lavish lifestyle, spending more than £600,000 on a luxury farm in Kent, which he fitted out with a swimming pool and hot tub, and £700,000 on motor vehicles including a Ferrari Spyder.
Foster also spent money on expensive koi carp breeding pools and withdrew almost £3m in cash.
He was convicted of unauthorised investment activity, deliberately concealing facts from investors and stealing investors' funds, following a seven-week trial.
Foster was found guilty of six counts of theft, relating to his use of funds, and eight charges under the Financial Services and Markets Act 2000.
After the convictions, Serious Fraud Office director Richard Alderman said: "I am very pleased with this verdict.
"This was a very complex investigation and the SFO was determined to bring justice for the many victims who lost their hard-earned savings to this Ponzi scheme.
"I would like to thank colleagues from Kent Police and the FSA (Financial Services Authority) who helped us with our investigation."
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