Bulger and his other top lieutenant in the Winter Hill gang, Stephen "The Rifleman" Flemmi, worked alongside Martorano and together they killed at least 19 people from the late 1960s to 2000.
Martorano has admitted his role in 20 killings and served a lenient 12-year prison sentence in exchange for cooperating against Bulger, who is standing trial on a 32-count racketeering indictment in federal court in Boston.
On Tuesday, Martorano recounted several killings during testimony, USA Today said. One took place 38 years ago this week when he said Bulger and Flemmi plotted to kill a Boston bar owner whose loose lips about a previous killing were seen as a potential liability.
Martorano said six gang members including Bulger and Flemmi met and decided to kill the bar owner, Eddie Connors in a gas station phone booth.
"I let them out" of the car, said Martorano, who was driving. "They walked up the hill. They went to the phone booth and shot Eddie."
He described another killing that missed its intended target and instead killed the wrong person.
The group was hunting for Al Notorangeli in March 1973 as payback for allegedly killing Paulie Folino, an associate of Italian mob boss and Winter Hill ally Jerry Angiulo. When they thought they had located Notorangeli in a car they opened fire only to discover it was being driven by someone else. Notorangeli wasn't in the car. The victim, Michael Milano, wasn't the intended target, Martorano said.
They left Milano's body in the trunk of a car and it was only discovered after a group of youths stole the vehicle and made the discovery.
Martorano is testifying against Bulger because he said he felt violated when he found out Bulger and Flemmi had been informants for the FBI for decades.