In 1848 he unsuccessfully sought election for the New South Wales Legislative Council seat representing the Town of Parramatta, but in 1849 he was elected the member for Port Phillip. That seat was abolished when Port Phillip separated as the Colony of Victoria in 1851 and Macarthur remained in the council as the member for the Pastoral Districts of Lachlan and Lower Darling before resigning in 1855. In 1854 he was a member of a commission for the colony's participation in the Paris International Exhibition of 1855, where his fluency in French was invaluable in resolving initial confusion over the areas allotted to the Australian colonies. A selection of Australian timber specimens from the Exposition are displayed at Camden Park. Shortly afterwards he was knighted. He return to Australia in 1857 and was appointed a member of the Legislative Council in 1864, but he never took a prominent part in politics and was more at home with his pastoral pursuits, having been given stewardship of his family's landmark pastoral property Camden Park. He was also an active in club life and served as the president of the Australian Club.