The department was informed about director Taylor Simonin's suspension last month on a company call, sources told the Journal.
The credit card issuer received subpoenas from the U.S. Department of Justice late October and is also being investigated by other federal agencies for pricing practices at its foreign exchange international payments (FXIP) unit.
The unit offers cross-border payments services mainly to small and middle market business customers in five countries, including the United States.
FXIP accounts for less than 1 percent of AmEx's total revenue and the company does not expect the investigation to hurt overall operations or results.
A spokesperson for AmEx declined to comment on the matter, saying "this is an ongoing review, we do not comment on any personnel issues, whether related to FXIP or not".
(Reporting by Diptendu Lahiri in Bengaluru; Editing by Shinjini Ganguli)