The union was founded in 1888 as the Cardiff and Penarth Coaltrimmers' Protection and Benefit Association, representing people who worked as coal trimmers at docks in South Wales.
In 1890, it gained representation on the new Trimming Board, where it agreed rates with ship owners and coal dock managers.
[1] David Jenkins described the union by this time as "by far the most influential labour organisation in Cardiff's dockland".
[1] As one of the two main unions of coal trimmers in the UK, in 1913 the union affiliated to the National Transport Workers' Federation, and its president, Joshua Thomas Clatworthy, was elected to its executive committee.
[1][4] The union affiliated to the Labour Party, and at the 1918 UK general election it sponsored its president as a candidate.