He was a respected scholar of the taxonomy and phytogeography of the vascular plant flora of temperate eastern North America.

Fernald coauthored the book Edible Wild Plants of Eastern North America in 1919–1920 with Alfred Kinsey, which was published in 1943.

His parents were Mary Lovejoy Heywood and Merritt Caldwell Fernald, a college professor at the University of Maine.

Fernald attended Maine State College for a year, but began working as an assistant at the Gray Herbarium at Harvard University when he was 17.

With James Franklin Collins he coedited the exsiccata work Plants of eastern Quebec, Bonaventure County (1905).