Purchas, whose background is obscure, rose steadily through the Cambridge municipal hierarchy and ultimately served three mayoral terms. His financial abilities were clearly well regarded, as he frequently served as an auditor to the corporation. However, there was some disquiet within the corporation at the manner of his election as an alderman in 1624. Despite the custom that aldermen were always nominated by their fellows, Purchas and another man were selected by the common council, which then proceeded to elect Purchas as mayor. Some of the aldermen thereupon complained to the city’s high steward, Sir Francis Bacon*, who referred the matter to the Privy Council. The Council ruled that although the election was illegal ‘there is no exception taken to the quality of the men’, who were therefore ordered to be raised to aldermanic status. Purchas was subsequently re-elected mayor.
Purchas’s involvement with Parliament began in 1614, when he was ordered by the common council to help choose the eight members of the corporation whose responsibility it was to determine the identity of the town’s parliamentary representatives.
Purchas drafted his will on his deathbed in November 1638, and a day later was buried at St. Clement’s church. His son-in-law Francis Sheldrake acted as executor after Purchas’ widow declined either to execute the will or to undertake the disposal of her late husband’s personal property.