Lawyer Sidney Powell pleaded guilty to reduced charges Thursday over efforts to overturn President Trump’s loss in the 2020 election in Georgia, becoming the second defendant in the sprawling case to reach a deal with prosecutors.

Ms. Powell, who was charged by the district attorney of Fulton County, Fani Willis, alongside Mr. Trump and 17 others with violating the state’s anti-racketeering law, entered the plea just a day before jury selection was set to start in her trial.

Ms. Powell pleaded guilty to six misdemeanors related to intentionally interfering with the performance of election duties. As part of the deal, Ms. Powell will serve six years of probation, be fined $6,000 and write an apology letter to Georgia and its residents. She also agreed to testify truthfully against her co-defendants at future trials.

The attorney was initially charged with racketeering and six other counts as part of a wide-ranging scheme to keep the Republican president in power after he lost the 2020 election to President Biden. Prosecutors say she also participated in an unauthorized breach of elections equipment in a rural Georgia county elections office.

The acceptance of a plea deal is a remarkable about-face for a lawyer who strenuously pushed theories about a stolen election in the face of extensive evidence to the contrary. If prosecutors compel her to testify, she could provide insight on a news conference she participated in on behalf of Mr. Trump and his campaign shortly after the election. 

Prosecutors will likely also be interested in learning about a White House meeting she attended in mid-December of that year during which strategies and theories to influence the outcome of the election were discussed. Mr. Trump reportedly considered naming Ms. Powell as special counsel to investigate election fraud. 

Ms. Powell was scheduled to go on trial on Monday with lawyer Kenneth Chesebro after each filed a demand for a speedy trial. Jury selection was set to start Friday. The development means that Mr. Chesebro will go on trial by himself, though prosecutors said earlier that they also planned to look into the possibility of offering him a plea deal.

A Washington-based lawyer for Ms. Powell, Barry Coburn, declined to comment on Thursday. A lower-profile defendant in the case, bail bondsman Scott Graham Hall, last month pleaded guilty to five misdemeanor charges. He was sentenced to five years of probation and also agreed to testify in further proceedings.

Prosecutors allege that Ms. Powell conspired with Mr. Hall and others to access election equipment without authorization and hired computer forensics firm SullivanStrickler to send a team to Coffee County to copy software and data from voting machines and computers there. 

The indictment says a person who is not named sent an email to a top SullivanStrickler executive and instructed him to send all data copied from Dominion Voting Systems equipment in Coffee County to an unidentified lawyer associated with Ms. Powell and the Trump campaign.