The last time the world heard from Dino Calabro was in 2012 when, swaggering and ruddy-faced, he appeared in court as a government informant and testified about the brutal murders he had once committed on behalf of his mob employers.

Shrunken and remorseful, Mr. Calabro was back in court on Friday, this time facing a possible sentence of life in prison for his crimes and begging a judge for mercy. His pleas in Federal District Court in Brooklyn were answered as the judge concluded that his assistance as a witness had helped to bring down scores of Mafiosi. He was sentenced to a term of 11 years.

The sentence capped an inglorious career in which Mr. Calabro, known as Big Dino, rose from a teenage mobster to a vicious captain in the Colombo crime family who, by his own account, had over the years killed at least eight people, including Ralph C. Dols, a New York City police officer whose mistake was marrying the former wife of the Colombo family leader Joel Cacace, Mr. Calabro’s onetime boss. After he was arrested in 2008, Mr. Calabro turned informant and eventually testified at Mr. Cacace’s trial and at the trial of Thomas Gioeli, the Colombo family’s “street boss.”

For the last five years, Mr. Calabro, now 59, has been in the witness protection program and has continued to help the government pursue the mob, federal prosecutors said. Though many of the cases that he worked on still remain secret, one of the prosecutors, James Gatta, told the court on Friday that Mr. Calabro had helped the government essentially “incapacitate” the Colombo clan.