Martyn first joined the Conservative Primrose League, but while working in Reading she lodged with her maternal aunt, Mrs Bailey, who held pronounced left-wing views. She briefly became a radical and then a socialist. In 1891, she was appointed a governess at the Royal Orphanage Asylum in Wandsworth, London, and joined the London Fabian Society. The following year, ill-health forced her to give up work and she began to devote herself full-time to the socialist cause. However, this was tempered by the devout religious views she had inherited from her parents, and she strongly disagreed with the Marxist principles of many of her contemporaries. For a while, in 1893, she was a sub-editor on the Christian Weekly.

Although she had many articles published in journals, Martyn was predominantly known as a lecturer. She became nationally recognised and large crowds turned up to hear her speak as she travelled round the country. In 1896, she was elected to the National Administrative Council of the Independent Labour Party and became Editor of Fraternity, the journal of the International Society for the Brotherhood of Man, and ILP trades union organiser for the North of Scotland. The work and travel, however, undermined her already-fragile health, and a few months later she contracted pneumonia while organising female workers in Dundee. She died a few days later at the age of 29 and was buried in Balgay Cemetery in the city. Keir Hardie wrote that she was the leading socialist of her day.