Despite the fact that Mary Macarthur’s father was a conservative, an anti-socialist and an opponent of trade unions, Mary, nonetheless, joined and became active in the Shop Assistants’ Union. In 1905 she, along with others, helped to launch the sweated trades’ exhibition and in 1906 formed the Anti-Sweating League. In the same year she formed, together with Margaret Bondfield (Shop Assistants’ Union) the organisation for which she is most renowned: the National Federation of Women Workers (NFWW).  Without the support of the NFWW, the strike among women employed at Millwall Food Preserving Factory, and those of the Cradley Heath Chainmakers and the Kilburnie netmakers would have been doomed to failure. Relief from their starvation wages and intolerable conditions was largely due to Macarthur's able championship of their claims.

During the First World War Mary Macarthur was active in the National Federation of Women Workers. Margaret Bondfield, wrote about her activities in a ‘A Life's Work’ (1948)Mary Macarthur had endeavoured for a long time to get a minimum wage ruling for a very large class of operatives in munitions work. In 1916 she had secured an award from the Munitions Tribunal for an increase in the wage rate for a large firm in the Newcastle area employing 8,000 women. Week after week went by, and still the firm was not given authority to pay the increased rate.

One morning Mary was rung up, and the furious voice of Winston Churchill, Minister of Munitions, asked her in effect what did she think she was up to, allowing the girls to stop work. Mary answered that the girls had waited patiently for the wages award granted them three months ago. She had not advised them to come out, and she would not advise them to go back until the firm was instructed not only to pay the rate, but promptly to pay the back money.

It was a stay-in strike, and the girls sat on their seats before the machines, knitting socks for soldiers.

Within twenty-four hours the authorization to pay the rates came to the firm and work was resumed.

Her husband Will Anderson, also an active trade unionist, died in the ‘flu epidemic of 1919.