Johnston was born in
Los Angeles to physician William M. Johnston and Dora Etta (Farnsworth) Johnston. He studied botany at
Pomona College and received his AB from
University of California at Berkeley in 1920, his MA from Berkeley in 1922, and his PhD from
Harvard University in 1925. After graduation, he conducted field work in Chile on a Sheldon Travelling Fellowship from Harvard and received a
Guggenheim Fellowship in 1932. He spent most of his career at Harvard University, starting out as an assistant in the
Gray Herbarium from 1922 to 1931 and gaining promotions to research associate at the
Arnold Arboretum in 1931 and associate professor of botany in 1938. From 1948 to 1953 he also worked as associate director of the Arnold Arboretum. He died suddenly at home in
Jamaica Plain,
Massachusetts, on May 31, 1960. Johnston was considered a leading authority on the family
Boraginaceae. He collected
algae,
pteridophytes and
spermatophytes and traveled widely to collect specimens in the Andes, Panama, the southwestern United States, and northern Mexico. His plant collections are housed in the
California Botanic Garden and the
Harvard University Herbaria. ==Honors==