His lawyers said he would appeal the verdict.
After five days of deliberations, the jury of four women and eight men found Bulger guilty of all but one of the 32 counts he faced.
They included 11 of the 19 murders for which he had been charged. The one count for which he was not convicted was an extortion charge.
At the age of 83, the white-bearded Bulger -- a fugitive for 16 years before his arrest in California in 2011 -- is likely to spend the last days of his life behind bars.
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Besides murder, Bulger -- seen by reporters giving a thumbs-up to members of his family as he was led out of the courtroom -- was accused of extortion, money laundering and arms trafficking.
His trial, which began June 4, featured chilling testimony from 63 prosecution witnesses.
It heard harrowing tales of teeth being pulled from the mouths of murder victims to foil their posthumous identification and the fatal strangulation of a mobster's girlfriend who "knew too much."
But the defence team was frustrated that the trial did not emphasise the relationship between Bulger and the US Department of Justice.
"I don't think you've heard the last word from James Bulger," added fellow defense attorney Hank Brennan.
Bulger refused to testify at his trial, saying it was "unfair and a sham" because it would not recognise what he claimed was immunity from prosecution given by federal agents.
Bulger has always denied having been an FBI informant, but close links between some FBI agents in Boston and Bulger's Winter Hill Gang in the 1970s and 1980s have been well documented.
Former FBI agent John Connolly is currently in prison after being convicted in 2002 of effectively becoming a member of the gang.