His research career was devoted to the study of developmental dyslexia as a constitutional disorder, likely to be "a form of aphasia", to the recognition that children with dyslexia have special education needs and that there should be a statutory obligation of schools to meet these, and, with his wife Elaine, to the development and evaluation of teaching methods to provide such support.
His first lectureship was at University College of North Wales, Bangor in the departments of education and philosophy in 1949.
In 1974 he founded the Bangor Dyslexia Unit (now the Miles Dyslexia Centre) to recruit teachers, to train them in multi-sensory methods of training, and to organise their work in the community, in collaboration with Gwynedd and Anglesey Local Education Authorities.
Following his clinical observations of many single cases of developmental dyslexia and the consequent identification of the more common aspects of the syndrome, he developed the Bangor Dyslexia Diagnostic Test, first published in 1982 and now translated into a number of languages including Welsh and Japanese.
He also wrote extensively on the nature of intelligence, linguistic philosophy, religion and science.