Port Line had its foundations in 1914 when the Commonwealth & Dominion Line was formed with the merger of Tyser Line, Star Line, Indra Line and the Anglo-Australian Steam Navigation Co. The ships were painted in Tyser Line colours of grey hulls, white superstructure and buff funnels. They were named as in Anglo-Australian with the prefix "Port".
In 1916 Commonwealth & Dominion Line was taken over by Cunard Steam Ship Company and the funnels changed to red with black top and three black bands of that Company. In 1936 the name was changed to Port Line.
Port Line, along with the Ellerman & Bucknall and the New Zealand Shipping Company took over the Canadian National Steamships Line’s Australian services, principally serving the Canadian east coast, in 1936. This concern had been established by the Canadian Government to utilise some of the cargo ships built in that country during World War I. However, this venture had not been a financial success. Port Line’s contribution to the new service was to immediately order three purpose-designed vessels which operated along with vessels from the other partners. This concern traded as the Montreal, Australia & New Zealand Line, which became abbreviated to the MANZ Line. The new ships were the Port Montreal (1937), Port Halifax (1937) and Port Saint Johns (1939). The Port Jackson built in 1937 set a design standard for future ships in Port Line and was considered by many to be the finest ship ever built for Port Line.