In October 1861 he let his name go forward for election for the Pensioner Settlements, an Auckland seat, which he represented in parliament from 1861 to 1866, when he retired from parliament. At that time, the colonial parliament still met in Auckland. There he sat alongside Thomas Russell who soon established the Bank of New Zealand, with the benefit of a parliamentary Act. The bank had early information about the discovery of gold in Otago, which was then transforming the colony's prospects and was soon to change its demography. Mason was commissioned to design the bank's premises in Dunedin and Wellington which profoundly affected his later life. It seems that early in September he visited the southern settlement and made the decision to relocate there. At the same time an advertisement saw him selling his house and chattels in Auckland. The transfer document shows him as a "Gentleman", ""late of Auckland but now of Dunedin"".