In 1902, Mary became president of the Scottish National District Council of the Shop Assistants’ Union.
In 1902, Mary Macarthur's leadership journey took a significant leap as she was elected president of the Scottish National District Council of the National Amalgamated Union of Shop Assistants, Warehousemen and Clerks. This role marked her emergence as a prominent leader within the trade union movement. Her presidency was not only a recognition of her capabilities and vision but also underscored the growing acceptance of women in union leadership—a pivotal era for women's roles in labor representation. Under her guidance, the council championed labor rights and represented retail workers' interests, laying a foundation for her future national prominence in the labor movement.