BOSTON — James (Whitey) Bulger, the mobster who terrorized South Boston in the 1970s and ‘80s, holding the city in his thrall even after he disappeared, was convicted Monday of a sweeping array of gangland crimes, including 11 murders. He faces the prospect of spending the rest of his life in prison.

The verdict delivers long-delayed justice to Mr. Bulger, 83, who disappeared in the mid-1990s after a corrupt agent with the Federal Bureau of Investigation told him he was about to be indicted. He left behind a city that wondered if he would ever be caught — and even if the F.B.I., which had been complicit in many of his crimes and had relied on him as an informer, was really looking for him.

“This was the worst case of corruption in the history of the F.B.I.,” said Michael D. Kendall, a former federal prosecutor who investigated Mr. Bulger’s associates. “It was a multigenerational, systematic alliance with organized crime, where the F.B.I. was actively participating in the murders of government witnesses, or at least allowing them to occur.”

The trial was the final reckoning for a man small in stature but large in legend, who ruined hundreds of lives and deepened the stain on Boston’s already corrupt federal law enforcement bureaucracy. Mr. Bulger was on the run for 16 years, until the authorities found him in 2011 in Santa Monica, Calif., with an arsenal of weapons and $822,000 in cash in the walls of his retirement apartment.