UCAC was established in Cardiff in 1940 by a group of teachers who saw the need for a teachers’ union with aims, policies and services that would correspond to the specific needs of teachers living and working in Wales.
One of those teachers was Mr Gwyn M. Daniel, UCAC’s first General Secretary. The following is a translation from the record in the Minutes Book of the inaugural meeting that established UCAC on 14 December 1940 at T?’r Cymry, Gordon Street, Cardiff:
"A good number of teachers of every grade from  Elementary School to University came together at T?’r Cymry, Gordon Street, Cardiff at 1.30 on Saturday, December 14, 1940 to establish the Union. 
Mr Gwyn M. Daniel, one of the promoters of the association from the start, was asked to chair the meeting. Letters were read from Mr Ifan ab Owen Edwards and Mr Saunders Lewis tendering their apologies but wishing every success to the Union. The message was the same from Miss Ellen Evans, Headmistress of Barry Training College".
This meeting was the culmination of a year of campaigning and correspondence.  A leaflet had been distributed in the name of UCAC to potential supporters noting that Undeb Cenedlaethol Athrawon Cymru was to be established in January 1940. The leaflet was signed on behalf of the organisation by Evan J. Jones, Griffith J. Williams, Gwyn M. Daniel and Victor H. Jones. It would only be fitting to add the names of Cassie J. Davies, Enid Pierce Jones, Isaac (Eic) Davies, D.J. Williams and Gwyneth M. Davies to these four as the founders of UCAC.
There was a good response to the leaflet and 105 people were invited to the  Inaugural Meeting. There is no formal record of who attended the meeting, but undoubtedly there was support for establishing UCAC as an organisation.
There was an organisation by the name of Undeb Athrawon Cymreig (Welsh Teachers Union) already in existence and the founders of UCAC were eager not to undermine the work of UAC in any way. This was the organisation that originally published 'Yr Athro'.
It is difficult to summarise the complete history of the Union here. As Rhodri Llwyd Morgan in his article establishing UCAC’s Archive 1940 – c.1980 (Amser Egwyl, Issue 1, December 2000) states:– "At the time of its establishment, UCAC was a pioneering organisation. UCAC was the first professional trade union to be established in Wales by Welsh people for Wales. It represented a body that insisted on concentrating minds on the educational and cultural needs of Wales".
Over the years since its establishment, UCAC has played an important role in many fields, including the history of education in Wales, unionism in Wales, and the social and linguistic history of Wales since 1940. 
Many documents that cronicle the history of UCAC are to be found in the UCAC Archive Collection 1940 - c.1980 at Cardiff University Library. The archive includes all the papers and documents that were kept at T?’r Cymry, Cardiff whilst the Union’s Head Office was situated there.  This includes all the papers that belong to the Union from its establishment in 1940 to the time of moving its Headquarters to Aberystwyth in 1983.  The archive also contains more recent papers.