Thomas Rees was an eminent preacher, a hymn-writer, a Biblical commentator (he translated Barnes's New Testament commentary in 1860, and himself published commentaries), and one of the founders of Yr Adolygydd, 1850, the first Welsh Independent quarterly. But he is remembered rather as the historian of Nonconformity and Independency in Wales; from an early age he had a passion for history. His History of Protestant Nonconformity in Wales (1861 — enlarged edition in 1883) is very well known; the original intention that Rees should co-operate with David Morgan (1779-1858) (q.v.) was abandoned, as Rees had no very high opinion of Morgan's work. In 1852, Rees had suggested to John Thomas (1821-1892) (q.v.) that they should jointly undertake a history of Welsh Independency. They came to agreement in 1862; publication began in 1870; and the work was completed in 1875, in four volumes (Thomas added a fifth in 1891). The work had been planned in such a way as to leave the older and more historic congregations to Rees, and the more modern to Thomas; but Rees's eyesight began to suffer, and in the event Thomas had to write more than half the work — including, e.g., the churches of Carmarthenshire, despite their long history and their great importance. It would seem that Thomas, rather than Rees, was chiefly responsible for the sententiousness which so frequently irritates the reader.

Rees has been severely criticized as a historian. In especial, he has been charged with denominational prejudice. And to be sure, he was an extremely zealous Independent; and although (by common testimony) he was a most kindly man and a willing co-operator with men of other churches, yet it would be untrue to credit him with the urbanity which marks the old Baptist historian Joshua Thomas (q.v.). But Rees's prejudices do not outweigh his major virtue (in an historian), his unusual command of sources. He scented these from afar, diligently burrowed in libraries and record-offices, tramped miles from one place to another to garner local information. In particular, he differed from his compeers in that he perceived the value of the ‘church books’ of the older Dissenting churches — books which had been greatly neglected, and indeed have in several instances disappeared since Rees used them, so that he is our only evidence for their contents.

His real weakness was not his prejudices, which are easily discounted. It was rather his almost comical inability to reproduce documents exactly. With no ill intent, but unfortunately with not a word of warning, he abridges, paraphrases, ‘sums up’; and as not everybody agrees with his interpretation of them, the sources have continually to be consulted afresh. Yet we must remember that he had had no training. And his work is even today indispensable. More: when we take into account the length of its chronological span, the breadth of its geographical ambit, its orderly arrangement, and (speaking generally) the accuracy of its details, Hanes Eglwysi Annibynol Cymru is still the best of our ‘standard’ Welsh denominational histories.

There is a Life (Cofiant) of Rees by his collaborator John Thomas, 1888.