He worked energetically to form labour unions and branches of the Independent Labour Party in north Wales, assisting in founding the Caernarfonshire Labour Council in 1912 and the North Wales Labour Council in 1914. He took a leading part in the debate on Socialism in Yr Herald Cymraeg in 1908, followed by the publication of his first book, Y Werin a'i Theyrnas (1910), which left its influence on a number of unionists and Welsh Labour members. He took a keen interest in adult education. He was a tutor of classes held by the Workers’ Educational Association in Caernarfonshire for a long period (1928-59), and he maintained an official connection with the society until his d. In 1944 he commenced the publication of Lleufer, the Workers’ Educational Association periodical which he edited until 1965. He gained a M.A. degree for a dissertation on ‘A study of a rural and maritime community in the nineteenth century, with special reference to the relation between agriculture and shipping’ (Liverpool; 1928), and an hon. M.A. degree of the University of Wales in 1960.

He was a very effective speaker and radio broadcaster and he wrote many articles, pamphlets and books on a wide range of political, literary and historical topics, including: Y Blaid Lafur a Dinasyddiaeth y Gweithiwr (1912), Y Cynganeddion Cymreig (1923), Y Ddinasyddiaeth Fawr (1938), Hen longau a llongwyr Cymru (1949), Cau'r tiroedd comin (1952), Llafur a senedd i Gymru: ysgrifau, llythyrau a sgyrsiau (1954), a biography of Silyn (Robert Silyn Roberts) 1871-1930 (1956), and Ann Griffiths a'i theulu (1963); also ‘Glendid iaith’, a weekly column on grammar in Y Faner (c. 1957-62). As a tribute to him he was presented with the volume, Ben Bowen Thomas (ed.), Lleufer y werin; cyfrol deyrnged i David Thomas, M.A. (1965), and his autobiography was published posthumously, Diolch am gael byw (1968). Some of his papers are kept at the National Library of Wales.

He m. 26 July 1919, Elizabeth Ann Williams, New Broughton (d. 1955 after a very long illness) and they had a son and a daughter. He d. at the home of his daughter, the widow of Herman Jones (see above), at 2 Pen-y-bryn, Burry Port, Carms. on 27 June 1967.