 In 1841, he was elected to the Legislative Assembly of the Province of Canada as one of two members for the city of Montreal as a Conservative member; he resigned in 1843 to protest the movement of the capital from Kingston to Montreal but was reelected in 1844.Although initially opposing the use of French in the assembly, he later supported a motion by Denis-Benjamin Papineau to reinstate the use of French as an official language in parliament.Moffatt helped develop the harbour facilities at Montreal, serving as chairman of the harbour commission and was also president of the Montreal Board of Trade from 1844 to 1846.