The star Atlanta rapper Young Thug pleaded guilty to participating in criminal street gang activity in a dramatic courtroom scene on Thursday, bringing his starring role in the longest trial in Georgia history to an unexpected conclusion after bumpy witness testimony complicated the state’s prosecution.

Young Thug, born Jeffery Williams, was matter-of-fact as he admitted to six counts, including possession of drugs and firearms. Prosecutors had described Mr. Williams in opening statements 11 months ago as “King Slime,” the fearsome leader of a pack that terrorized the streets of Atlanta via gang warfare, robbery and drug dealing for nearly a decade as his music career took off.

His guilty plea on Thursday followed a tense courtroom moment in which the judge in the case, Paige Reese Whitaker, asked Mr. Williams if he was ready to accept a non-negotiated plea, instead of a negotiated deal with prosecutors, because of an impasse over sentencing. Mr. Williams, looking stricken, conferred with his lawyers briefly before the judge called a recess to allow him to decide.

In a non-negotiated plea, the judge is responsible for deciding the sentence based on recommendations from both sides.

Upon returning, Mr. Williams, 33, said he would accept the blind plea; he also pleaded no contest to two additional counts, leading a criminal street gang and conspiracy to violate the RICO act, the state’s racketeering law. The state said it would recommend a sentence of 45 years, with 25 served in custody, followed by 20 years on probation.

Mr. Williams’s plea came as his two remaining co-defendants indicated that they planned to continue at trial. Deamonte Kendrick, known as the rapper Yak Gotti, and Shannon Stillwell, known as SB or Shannon Jackson, stand charged with a 2015 murder, in addition to other counts, that prosecutors said kicked off a violent yearlong back-and-forth between two local factions of the national Bloods.