J. Bryan Williams, 63, entered the plea Thursday in U.S. District Court in Manhattan to charges of conspiracy and tax evasion, agreeing to serve at least three years and two months in prison with a maximum of four years and nine months in prison.
Judge Harold Baer set sentencing for Sept. 18, when Williams will also face potential fines and restitution of millions of dollars.
During the plea, Williams said he did not disclose the payments he received to Mobil Oil. A prosecutor two months ago had said Mobil Oil was a subject of the government's ongoing investigation in the case.
Williams, who lives in McLean, Va., remains free on $250,000 bail. He and his lawyer, David Schertler, declined to comment outside court.
Williams was manager of government crude acquisitions and sales before he retired in 1998.
During the plea, Williams said he opened two bank accounts in Switzerland in 1993 in the name of a British Virgin Islands corporation. He said he used the accounts between 1993 and 2000 to hide more than $7 million from the Internal Revenue Service.
He said the money included a $2 million payment he had received from people, organizations or governments he did business with on behalf of Mobil Oil.
"I knew what I did was wrong and unlawful," he said. "I also believe I acted in concert with others ... to escape detection by the IRS."
Outside court, Exxon Mobil lawyer Ted Wells said the company "had no knowledge of the $7 million that he received and hid in secret bank accounts." Mobil Oil and Exxon merged in 1999.
"Exxon Mobil has no knowledge of the reasons he received these secret payments," Wells added.
He said the Irving, Texas-based company was responding to all requests from the government but was not a target of the probe.
During the plea, Williams did not mention James H. Giffen, a New York businessman accused of making more than $78 million in unlawful payments to two senior Kazakh officials.
An indictment against Giffen, of Mamaroneck, N.Y., accuses him of accepting payments from the officials to secure oil contracts, defrauding Kazakhstan out of millions in oil deal dollars.
Giffen's lawyer, William Schwartz, said outside court Thursday that his client "has been indicted for acts he vehemently denies, and we expect he will be exonerated at trial."
Prosecutors said one Kazakh official used bribe money to buy more than $180,000 in jewelry and to pay for a Swiss spa visit while the other official used at least $45,000 to send his daughter to an exclusive Swiss high school.
By Larry Neumeister