One of the 19 defendants in a Georgia racketeering case against former President Donald J. Trump and his allies pleaded guilty on Friday to five misdemeanor charges, under a deal with prosecutors in which he would receive five years of probation.

The guilty plea of Scott Hall, 59, a Georgia bail bondsman, was a significant victory for Fulton County District Attorney Fani T. Willis, who secured an agreement from Mr. Hall to testify against other defendants. As of Friday afternoon, no other defendants have taken pleas; two of them, the lawyers Kenneth Chesebro and Sidney Powell, are scheduled to be tried together starting next month after demanding speedy trials.

Mr. Hall had been charged with racketeering, as well as six other felonies, for helping to carry out a data breach at the elections office in rural Coffee County, Ga., in January 2021. He and other Trump allies were apparently looking for evidence of ballot fraud after Mr. Trump lost the 2020 presidential election in Georgia.

Appearing in a Fulton County courtroom on Friday afternoon, Mr. Hall, wearing a dark suit and open-necked shirt, pleaded guilty to five misdemeanor counts of intentional interference with performance of election duties.

Under the terms of the deal, Mr. Hall is to pay a $5,000 fine, surrender his firearms carry license, perform 200 hours of community service and write a letter of apology to the people of Georgia. He is not to participate in any activities related to the administration of elections, and he agreed to testify truthfully in all further proceedings in the case.

Asked if he understood the terms, he told Judge Scott McAfee of Fulton County Superior Court, “I do, sir.”