The federal jury, however, acquitted Mandell on charges of plotting to kill Anthony "Tony Q" Quaranta as part of an effort to take over a strip club said to be connected to organized crime, the Chicago Tribune reported.
Mandell, 63, was tried on charges of conspiracy to kidnap, extortion conspiracy, murder-for-hire, obstruction of justice and weapons charges.
The jury heard undercover FBI recordings in which Mandell talked with an alleged accomplice about the plots.
Mandell's attorney, Keith Spielfogel, told jurors the audiotapes were "ridiculous" and said prosecutors did not present proof beyond a reasonable doubt, the Tribune reported.
Mandell had been on death row for a 1990 murder but was exonerated.
The jury Friday convicted him of planning to kidnap Campbell and use extortion to take his money and property before killing him and disposing of the body by chopping it up.