Thomas Menino's action, 70 years after the two self-confessed anarchists, Nicola Sacco and Bartolomeo Vanzetti, went to the electric chair for murder and armed robbery, marks the latest attempt by the city to lay the affair to rest.
Sacco, a cobbler, and Vanzetti, a fish pedlar, were tried and sentenced for robbing a paymaster and his guard at a time of fierce anti-immigrant sentiment in the US. The fairness of the judge was questioned.
Twenty years ago, on the 50th anniversary of the execution, the then governor of Massachusetts, Michael Dukakis, described the event as "a classic example of a miscarriage of justice" and said there was a strong case for believing one or both men were innocent. This week, Mr Menini, Boston's first Italian-American mayor, said he was accepting the plaque because it represented an important part of American history.