See his letters (ed. by C. Brooks and D. N. Smith, 6 vol., 1944-61).
He was born in South Shields, the son of a sea captain from Narberth with whom the family often travelled. They moved to Cardiff during the 1890s, and Captain Thomas died at sea in 1897. He began work in a shipping office, but changed to a career in architecture on advice from a phrenologist. In 1903 he won the architecture competition at the National Eisteddfod of Wales in Llanelli. After several years working in England, he began collaborating with Ivor Jones of Cardiff, and they went into partnership in 1913. In 1915, he joined the Artists Rifles and served on the Western Front. When World War I finished, he returned to Cardiff. He was commissioned by David Davies, 1st Baron Davies of Llandinam, to design the Temple of Peace in Cathays Park.
As a designer of civic buildings, such as Swansea Guildhall (1930-34), and Hiatt Baker Hall (1966) he was a rival to Arthur John Hope. Other works by him included the campus of University of Wales, Aberystwyth. He was awarded the Royal Gold Medal in 1939, and was knighted in 1946.