When Western Australia introduced representative government in 1870, he ran unsuccessfully for the Legislative Council seat 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, "[u]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.

[2] In 1894, some newspaper editors accused him of a conflict of interest because of his dual roles as minister and leading business investor.