Randy Teter, 38, will serve another 30 years in state prison, Special Circuit Judge Gael Wood announced Tuesday.
Wood normally handles cases in the 20th Circuit — Osage, Gasconade and Franklin counties.
During a five-minute hearing Tuesday morning, Wood told Teter: "It's clear you've had a rough life — but there's no excuse for what you did."
Teter pleaded guilty Jan. 29 to second-degree murder for the August 2014 assault of his cellmate at the Jefferson City Correctional Center.
Court records show the victim, Mark Melton, 35, was severely beaten — including being stomped on his head and throat.
At the time of Melton's murder, Teter was serving a 22-year sentence for a Jackson County conviction of second-degree murder and armed criminal action.
After the assault, Teter was moved from Jefferson City to the Potosi Correctional Center at Mineral Point.
After the assault, Melton was taken to University Hospital in Columbia, where he died a few days later.
At the time of his death, Melton was serving a nine-year sentence after pleading guilty in Dunklin County — in the Bootheel — to a December 2007 charge of attempted first-degree sodomy.
Teter's new, 30-year sentence is to be served at the same time as the previous sentence.
Teter's public defenders — Robert Lundt and Srikant B. Chigurupati, both from St. Louis — had urged Wood to impose only an additional 15 years.
They argued in a written "sentencing memorandum" that Teter had not "knowingly" killed Melton and that mitigating factors in his case included that he had suffered from "a terrible childhood" that involved physical and sexual abuse by his mother.
Tuesday's hearing followed a two-day sentencing hearing in April, where Teter's public defenders and Cole County Prosecutor Locke Thompson and Assistant Prosecutor Scott Fox presented evidence for Wood to consider in reaching his final sentencing decision.
Wood reminded Teter he had 180 days to file any appeal in his case.