Mass. Gen. Laws ch. 54 § 144
Upon failure to choose a county commissioner, the board of examiners shall forthwith issue precepts to the aldermen of each city and to the selectmen of each town in such county, directing them to call an election to elect such officer on a day appointed therein.
Upon a vacancy by removal or otherwise in the office of county commissioner, the board of examiners shall in like manner issue precepts for an election to fill such vacancy at the next biennial state election for which precepts can be seasonably issued, unless the term of office of the commissioner whose office is so vacant expires on the first Wednesday of January following such election.
Also upon such vacancy, the two remaining county commissioners and the clerk of the courts for the county, or a majority of them, may appoint a person, not a resident of the same town as either of the remaining commissioners, to fill the office of county commissioner until a person is elected thereto and qualified.
Mass. Gen. Laws ch. 54, § 144