Martorano told jurors he'd admitted to killing 20 people in Massachusetts, Florida and Oklahoma on behalf of Bulger and his top lieutenant Flemmi, a/k/a "The Rifleman."
Martorano was sentenced to 14 years in prison as part of a plea deal in exchange for his cooperation against Bulger and Flemmi but Martorano served just 12 years for his crimes, he told jurors Monday.
The Boston Globe said Martorano described Bulger and Flemmi as "my partners in crime, my best friends, my children's godfathers."
Bulger faces a 32 count federal racketeering indictment filed in 1997 though Bulger avoided being taken into custody for years.
Martorano said he was shocked to learn Bulger and Flemmi had been working as informants for the FBI, providing information on a number of criminal underworld elements. They were "handled" by FBI agent John Connolly, who tipped off Bulger to ongoing investigations in exchange for information on his dealings with other mobsters.
Martorano said he's testifying against Bulger because his cooperation with the FBI broke the trust they shared while running the Winter Hill gang.
The whole plot was immortalized in Martin Scorcese's mafia flick, "The Departed."
Martorano is serving as the prosecution's star witness, recounting numerous killings and other crimes he, Bulger and Flemmi committed together from the late 1960s to 2000. His testimony is expected to resume in federal court Tuesday.