Mark H. Long, Mayor Vincent Gray's campaign chauffeur, leaves the courthouse after a hearing in Washington on Friday. (Nikki Kahn/The Washington Post)

D.C. Mayor Vincent C. Gray’s onetime campaign chauffeur admitted in court Friday that he worked with others to violate campaign finance laws during the 2010 race.

Mark H. Long, 47, pleaded guilty to conspiring with D.C. businessman Jeffrey E. Thompson and others to conceal under-the-table campaign payments and participating in a scheme to pay off another mayoral candidate to exit the race.

In March, Thompson pleaded guilty in federal court to funding a “shadow” effort to help Gray (D) win the 2010 mayoral election by pumping more than $660,000 into the campaign. Long becomes one of the closest campaign aides to the mayor to be linked with allegations of accepting illegal funds from Thompson. He worked as Gray’s personal driver and served as his head of advance operations during the 2010 campaign.

Gray, who is traveling in China, has denied wrongdoing since the investigation began more than three years ago. Robert S. Bennett, his attorney, declined to comment Thursday, when the charges against Long were made public.

“Long is now the sixth person associated with the 2010 mayoral campaign to become a felon as a result of illegal conduct on behalf of that campaign,” U.S. Attorney Ronald C. Machen Jr. said in a statement. “Six months ago, when contractor Jeff Thompson walked into court and pulled back the curtain on widespread corruption in D.C. politics, we pledged to continue our work to hold accountable everyone who conspired with him to poison our political system. . . . Our work continues.”

New information about Long was revealed during the 20-minute hearing in D.C. Superior Court. Assistant U.S. Attorney Michael Atkinson told Judge Anita Josey-Herring that Long drove Jeanne Clarke Harris, a D.C. business consultant and Thompson associate, to a meeting during which Harris gave a “package” to a local union official. That union official, who was not named, endorsed Gray during the 2010 primary, prosecutors said. Details of the package were not disclosed. Long declined to comment after the hearing.

Sentencing for Long was delayed because prosecutors said he was cooperating in their investigation. Under the plea agreement, he faces a sentence ranging from probation to a year in jail.

The Washington Post reported last year on discussions involving Gray, Harris and Leo Alexander, another candidate in the 2010 primary, about arranging for Alexander to drop out of the race before the election. The charges­ against Long include prosecutors’ first mention of those conversations, including another “secret meeting” in Maryland between Gray and Alexander that Long and Harris allegedly arranged.

The Post also reported in 2012 that Long and the SUV in which he chauffeured Gray were funded not through the Gray campaign but through Harris, who pleaded guilty to wrongdoing in 2012. The charging papers filed Thursday say Harris’s consulting firm — using funds Thompson provided — paid Long’s salary and for the SUV.

Before joining Gray’s campaign, Long in 2008 made a failed run for an at-large seat on the D.C. Council. That campaign, according to the charging papers and people familiar with the federal investigation, was one of several Thompson secretly funded in violation of campaign finance laws.

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