Mass. Gen. Laws ch. 34, § 4: Election; tenure

Section 4. Except in Nantucket and Suffolk counties, there shall be elected in each county, as provided in section one hundred and fifty-eight of chapter fifty-four, for terms of four years beginning the first Wednesday of January following their election and until their successors are qualified, three county commissioners, who shall be sworn before entering upon their duties, and shall annually, at their first meeting after the first Wednesday of January, choose by ballot a chairman. In Nantucket county the selectmen of Nantucket, and in Suffolk county the mayor and city council of Boston, the municipal council of Chelsea and the city council of Revere, in their respective cities, and the selectmen of Winthrop in said town, shall, except as otherwise provided, have the powers and perform the duties of county commissioners.