He returned to Mount Perry in 1899 to work in the copper smelter and later farmed in the Burnett River district for two years.

[1] Martens also worked at the Pleystowe Sugar Mill west of Mackay for a period, where two of his co-workers were future Prime Minister Arthur Fadden and Queensland state politician Maurice Hynes.

In 1928, he was elected to the Australian House of Representatives as the Labor member for Herbert, defeating the sitting Nationalist Lewis Nott.

During World War II, Martens opposed moves towards mass internment of enemy aliens, with his electorate including a significant Italian-Australian population.

[3] In 1942 he wrote to Prime Minister John Curtin that he was "astounded to see what was taking place in regard to internments" in North Queensland, citing examples of Italian internees who were naturalised citizens, had lived in Australia since childhood, were members of the Volunteer Defence Corps, or had sons serving in the Australian military.