In 1920–21, they joined the Welsh Section of the Southern League and finished runners-up in their first season.

[1] Despite a promising start in league football, finishing 8th in the 1921–22 season, Aberdare suffered financial instable throughout the 1920s.

In the 1926–27 season, Aberdare Athletic finished bottom of the Third Division South and failed to gain re-election to the League, with Torquay United taking their place.

That round of voting was marred by controversy as the potentially tie-breaking ballot was reported to be "spoilt" and Aberdare's secretary claimed that one of the scrutineers was "an interested party".

[5] The club now fully adopted the name "Aberdare & Aberaman Athletic", and rejoined the Southern League.