In 1890 she moved to Sydney, where she became part of the "Bulletin school" of radical writers - the greatest influence on her work was Henry Lawson, but it was A G Stephens, literary editor of The Bulletin, who published her verse and established her reputation as a fiery radical poet, champion of the workers and the oppressed.
In 1896 she sailed with William Lane and other socialist idealists to Paraguay, where they established a communal settlement called New Australia. There she married William Gilmour. By 1902 the socialist experiment had clearly failed and the Gilmours returned to Australia, where they took up farming near Casterton in the Western District of Victoria.
Gilmour's first volume of poetry was published in 1910, and for the next fifty years she was one of Australia's most popular and widely read poets, although advanced literary opinion held much of her verse to be doggerel and propaganda. In 1908 she became women's editor of The Worker, the newspaper of Australia's largest and most powerful trade union, the Australian Workers Union (AWU). This gave her a platform for her powerful journalism, in which she campaigned for better working conditions for working women, for children's welfare and for a better deal for the Aboriginal people.
By 1931 her views had become too radical for the AWU, but she soon found other outlets for her writing. Later she wrote a regular column for the Communist Party's newspaper Tribune, although she was never a Communist. Despite her politics, she accepted an imperial honour in 1937 and became Dame Mary. During World War Two she wrote stirring patriotic verse such as No Foe Shall Gather Our Harvest.
In her later years Gilmour, separated from her husband and enjoying her status as a national literary icon, lived in Sydney. Before 1940 she published six volumes of verse and three of prose. After the war she published volumes of memoirs and reminiscences of colonial Australia and the literary giants of 1890s Sydney, thus contributing much material to the mythologising of that period. She died aged 98 and was accorded a state funeral.