John Vincent Martorano was born on December 13, 1940, in Somerville, Massachusetts. The son of Angelo "Andy" Martorano, a Sicilian immigrant from Riesi, and Elizabeth Mary "Bess" Hunt, of Irish descent, Johnny was raised in a Catholic household. His early years were spent in East Milton, a neighborhood with a strong Irish-American presence. He and his brother, James “Jimmie” Martorano, were raised with traditional values but were no strangers to the street codes of Boston's underworld.

Johnny was a standout athlete during high school, co-captaining the football team at Milton High in 1958. Despite receiving scholarship offers from several colleges, Martorano declined them, choosing instead to stay in Boston. He had a strong bond with his father, who instilled in him a sense of family loyalty — one that, unfortunately, came hand-in-hand with exposure to organized crime. According to Johnny himself, his father once told him, “You’re the oldest son and this is your heritage… you’ve got to take care of your family and be a man.”

After high school, Martorano drifted into Boston’s notorious Combat Zone. It was there that he came under the wing of Stephen Flemmi, a key figure in the Winter Hill Gang.

Martorano's first known hit took place in 1964, when he allegedly killed Robert S. Palladino, a made member of the Patriarca crime family, who was set to testify in a case involving the murder of a prostitute. That same year, he gunned down William “Billy” O'Brien in a drive-by attack on Morrissey Boulevard, shooting him 17 times and injuring fellow mobster Ralph DeMasi.

As an enforcer for the Winter Hill Gang, Johnny became known as the gang’s most reliable killer. In January 1968, he murdered three people — Hubert "Smitty" Smith, Elizabeth Dickson, and 17-year-old Douglas Barrett — in one sitting after a dispute involving Flemmi. Despite not knowing the full context, Martorano demanded “respect” from Smith and, feeling disrespected, executed all three in a parked car in Dorchester.

In another chilling case of mistaken identity, Martorano killed Michael Milano on March 8, 1973. Mistaking Milano for rival gang leader Al Notarangeli, he opened fire with a machine gun while Milano was driving through Brighton, wounding two others. One of them, Louis Lapiana, was left paralyzed for life.

Martorano's activities weren’t limited to Boston. He was linked to the 1981 assassination of businessman Roger Wheeler in Oklahoma and the 1982 murder of John Callahan in Miami. Both hits were linked to organized crime and were carried out at the request of Whitey Bulger and Stephen Flemmi. Although Martorano initially objected to killing Callahan, he eventually carried out the murder.

When Martorano found out in 1979 that he was about to be indicted for a horse-race fixing scheme, he fled and spent 16 years as a fugitive in Florida. In 1995, he was arrested in Boca Raton and indicted along with other major players, including Flemmi. Furious at being double-crossed by his former associates, Martorano agreed to cooperate with federal prosecutors in 1999, striking a plea deal in exchange for his testimony and confession to 20 murders.

He served only 12 years in prison and was released in 2007, receiving $20,000 to start over. He refused witness protection and settled back in Boston. Martorano’s testimony would later play a key role in the trial of James “Whitey” Bulger in 2013.