The matter relates to the period when Mandaric was in charge at the Premier League club, and is believed to centre around transfers involved midfielder Amdy Faye.
Mandaric completed a takeover at the Walkers Stadium earlier this year.
A spokesman said: "I can confirm that on Wednesday morning Mr Mandaric was asked to assist City of London Police with their enquiries into a matter dating back to 2003 at his former club.
"Mr Mandaric made himself readily available and full co-operated with the Police enquiry.
"While the investigation remains on-going, Mr Manadaric will continue to offer his full and total support to the Police and will refrain from making any further public statement at this time."