Lance Poulsen was convicted on 12 counts in all by a federal jury in Columbus, Ohio, Bloomberg said.
Prosecutors argued that he had sold bonds with the promise of buying accounts receivable but in fact left many investors with a "worthless IOU". Around $2.2 billion was advanced to companies where Mr Poulsen held shares, until National Century filed for bankruptcy.
The news service noted that the collapse of the firm contributed to the bankruptcy of around 275 hospitals, clinics and other health care establishments.
Mr Poulson now faces 30 years to life in prison. The 65-year-old is already serving a ten-year sentence after being convicted of trying to bribe the prosecution's main witness. His legal team said he plans to appeal.
In court, Justice Department trial attorney Leo Wise said: "This is one of the largest frauds the FBI has ever investigated."
According to the Associated Press, seven National Century executives have been charged in connection to the fraud.