Brynley retired from his post as Librarian of the National Library of Wales in 1994. He was previously Professor of Welsh Language and Literature at Swansea University and had also been on the staff of the Welsh Department at Aberystwyth University. Since his retirement he has continued his researches in medieval Welsh historical literature and the life and work of Edward Lhwyd at the University of Wales Centre for Advanced Welsh and Celtic Studies where he is an honorary Research Fellow.

His other major activity is as the current Editor of the Dictionary of Welsh Biography, the important on-going contribution made by the Society since 1953, with the cooperation of the National Library, to the cultural and intellectual life of Wales.