He faces up to 15 months in prison and $10,000 in fines. Sentencing was set for March 19.
The plea, the same day an indictment was filed, came only two days after his lawyer said Duke had returned to Louisiana after three years out of the country to negotiate with prosecutors.
Duke was accused of filing a false 1998 tax return. The mail fraud charge accuses Duke of "obtaining a substantial sum of money" through mail solicitations and misusing the money.
Prosecutors were not available immediately to detail the charges.
Duke's attorney, Jim McPherson, said Monday that the Justice Department had been examining Duke for possible income tax violations involving the $100,000 sale of a list of Duke supporters to Gov. Mike Foster in 1995.
Duke had just started a speaking tour in Russia in January 2000 when federal agents raided his home in Mandeville, La. A search warrant, based on testimony from confidential informants, alleged that Duke took hundreds of thousands of dollars he solicited from supporters and gambled the money away at casinos.
Until his return late last week, Duke had been lecturing and speaking in Russia and other European countries in what became a crusade for "white survival" against Jews and non-Europeans.
Duke won a Louisiana House seat in 1988 and lost runoffs for the U.S. Senate in 1990 and governor in 1991, while claiming to have jettisoned his racist views.
He came in third in a 1999 congressional race in the New Orleans area.