At this time Mason was still a member of the house of representatives and on 21 July 1865, he was elected the first Mayor of Dunedin of the newly incorporated City of Dunedin, defeating four other candidates. He retired from parliament in 1866 and from the mayoralty in 1867. He was a cognisant, though not outspoken, member of the house. In the civic chair he presided over improvements to the city's drains and the levelling of streets. His Bank of New South Wales in Princes Street was completed in 1866, a refined, three storey masonry building. Recessed from the street and ornamented with gas lamps and pillars, it won high praise and a careful description from the "Otago Daily Times". Its felicitous street front was removed in the 1970s.