Undeb Cymru Fydd's main activity was to press the UK government to adopt policies specific for Wales and to promote the Welsh language in areas such as education and broadcasting.
In addition, a series of books and magazines such as Atgofion Cymru (Welsh Memories) for Welsh-speaking members of the armed forces during World War II.
A joint Committee between Undeb Cymru Fydd and the Churches was appointed to address the displacement of factory workers by the civilian wartime service during World War II.
In 1950 a special conference was held which resulted in the Parliament for Wales Campaign culminating in a petition containing 250,000 names being presented to the UK Government.
Throughout its existence, Undeb Cymru Fydd campaigned for Welsh-medium schools, the establishment of an Education Committee, and for better and more Welsh language provision in the media.