Perhaps ambition called again. He was offered an appointment as "Superintendent of Works" to the nascent government of William Hobson, Lieutenant Governor designate of New Zealand, which he accepted, sailing to join Hobson at the Bay of Islands where he arrived on 17 March 1840. He thus became the first professionally trained architect in New Zealand. His title was officially "Superintendent of Public Works" but he maintained he had been appointed "Colonial Architect". He adopted that title which was used by government officers in addressing him. He went to assist with establishing the new capital, Auckland, in September 1840. There he oversaw the erection of the prefabricated first Government House before resigning in 1842 and going into partnership with Thomas Paton. Formerly the government postmaster Paton had also resigned and the two men set up as auctioneers and architects. Mason designed the church of St Paul which was started in 1841. He had bought land and now built on it without making a fortune, put up premises for the New Zealand Banking Company and designed houses. He became involved in other commercial affairs but in 1841 his nine-year-old son was drowned in a well. The boy may have been murdered and his parents were deeply distressed.