Fotios Geas, a convicted Mafia hitman, was serving life in a federal prison in West Virginia when, prosecutors say, he rained lethal blows on the head of the Boston crime boss James (Whitey) Bulger while two other inmates acted as lookouts.

On Friday, Mr. Geas, 57, was the last of the three men — who had all agreed to plea deals several months ago — to be sentenced in the 2018 murder, receiving an additional 25 years. Mr. Bulger, a sickly 89-year-old gangster widely known to have been an F.B.I. informant, was bludgeoned to death in his cell less than 12 hours after he was transferred to the lockup in Bruceton Mills, W.Va.

At a hearing in U.S. District Court in Clarksburg, W.Va., Mr. Geas, a New England associate of the Genovese crime family already serving time for his role as an assassin and enforcer, pleaded guilty to voluntary manslaughter and assault resulting in serious bodily injury.

U.S. District Judge Thomas S. Kleeh sentenced Mr. Geas to 15 years on the manslaughter charge and 10 years on the assault charge. But the additional time will have no practical effect on how long Mr. Geas spends behind bars because of his life sentence after his conviction in a double-murder and racketeering trial in 2011.

In the Bulger case, before his plea, Mr. Geas had been charged with first-degree murder, conspiracy to commit first-degree murder and assault resulting in serious bodily injury. He was also charged with first-degree murder by a federal prisoner serving a life sentence.

Prosecutors said that Mr. Geas used a lock attached to a belt to pummel Mr. Bulger just hours after Mr. Bulger had been moved from a Florida lockup where he had been serving two consecutive life sentences for his role in 11 murders.

But at Friday’s hearing, which lasted about two and a half hours, one of Mr. Geas’s lawyers, Nathan Chambers, disputed the claim in the pre-sentencing report that a lock was used in the beating. Mr. Chambers said that Mr. Bulger was struck with his fist. The distinction, he said, is important because it will have a bearing on how Mr. Geas is treated in prison.

“There is no evidence that a padlock was used either inside a sock or attached to a belt,” Mr. Chambers told the court, adding that Mr. Bulger’s injuries showed “nothing that corresponds to being bludgeoned with a lock.”

He noted that the medical examiner determined that Mr. Bulger’s death was the result of a single blow to the left ear, likely from a fist.

Mr. Chambers said that the story about the lock originated from an inmate seeking reduced time in prison and was not credible. Ultimately, Judge Kleeh agreed to note Mr. Geas’s objections in the pre-sentencing report.

Kitty Bennett and Kirsten Noyes contributed research.