Ernest married high school teacher Ethel Highwarden on June 26, 1912, and together they had three children, Margaret, Highwarden, and Maribel, before divorcing in 1939. The very same year, Ernest married a woman by the name of Hedwig Schnetzler, and had a daughter by the name of Elisabeth. His parents were Charles Frazier and Mary Matthews Just.

Ernest received his Ph.D. in 1916, becoming one of the first African Americans to do so.

He wrote a number of well-received articles on experimental methods that was later collected in a volume entitled Basic Methods for Experiments on Eggs of Marine Animals (1939).

Just wrote a book with Hedwig Schnetzler, his second wife and research assistant, entitled The Biology of the Cell Surface (1939.)