In collaboration with the Welsh Plant Breeding Station between 1919 and 1939 Fagan became one of the leading British scientists studying the chemistry of grass and its conservation. He was a pioneer in this field and his articles, most of which were published in the Welsh Jnl. of Agric., bear testimony to his ability, dedication and leadership as an agricultural scientist. His meticulous and accurate analyses were of inestimable value to plant breeders, and according to R.G. Stapledon (see below) Fagan laid sure foundations for understanding the innumerable factors affecting the nutritive value of grasses, clovers, and other grassland plants. He continued his researches to the end of his life, but did not receive the commendation which was due to him for his pioneering work, possibly because he was by nature unassuming, and reluctant to extol the value of his own researches.

He m. Helena Teresa Hughes, and they had one son. Fagan d. in Aberystwyth, 10 Feb. 1951, and was buried in the town cemetery.