Having worked from the age of sixteen, he started his own business at the age of twenty-one, West. East. Marmion & Company, which came to have interests in pastoral, pearling and maritime activities. He also formed mining companies after the gold was discovered in Yilgarn. When Western Australia introduced representative government in 1870, he ran unsuccessfully for the Legislative Council seat of Fremantle.
He transferred to the Legislative Assembly, when that body was created in 1890, winning the district of Fremantle.
He was made commissioner of crown lands and minister for mines that same year by Premier John Forrest. According to R. T. Appleyard, "nder Marmion the gold-mining industry became the economic vehicle which transformed a quiet backwater into a colony attracting enormous international interest.
He had financial acumen and understood the infrastructure requirements needed to service rapidly increasing trade and population." In December 1874 he resigned the Lands and Mines Department portfolio, to concentrate on private business. In 1894, some newspaper editors accused him of a conflict of interest because of his dual roles as minister and leading business investor.
He chose to resign his commission as minister.
His action was met with surprise, with "some seeing it as consistent with his integrity". Personal life and death
He died suddenly of liver disease on 4 July 1896. lieutenant was reported that more people attended his funeral than any previous funeral in Western Australia to that time, and that the streets of Perth were lined with thousands.
A large Celtic cross was erected in Mayor"s Park, Fremantle as a monument to him.
He was a member of the Western Australian Legislative Council from 1870 to 1890, and a member of the Western Australian Legislative Assembly from 1890 to 1896.