Thomas returned to Wales in 1940 and took teaching positions, first at
Ebbw Vale and then
Brynmawr. Having joined
Plaid Cymru as a student in Cardiff, Thomas remained active in the party throughout the 1940s and 50s and fought to promote more Wales-focused history in Welsh schools and for the use of the Welsh language in education. Thomas returned to Cardiff and academia in the 1960s, being appointed director of the newly-established Language Research Unit in the Welsh Department at
Cardiff University. During her time at the Research Unit the number of speakers of the dialect of south-east Wales,
Gwenhwyseg was diminishing significantly. As such, Thomas gave the study of this dialect special attention, and the period of her directorship saw the creation of a corpus of work which would make a notable contribution to the study of the Welsh language. == Nantgarw ==