Harold St. John was a professor of botany at the University of Hawaiʻi at Mānoa from 1929 to 1958. A prolific specialist in field botany and systematics, he is credited with discovering about 500 new species of Pandanus, along with many other species, especially in the Pacific Islands.
Born in Pittsburgh, Pennsylvania, he was educated at Harvard University, where he earned a Ph.D. in 1917. After service in Europe during World War I, he taught botany at the State College of Washington, where he also became the curator of its herbarium. In 1929, he joined the faculty of the University of Hawaiʻi, where he served as longtime chair of the botany department, then as director of the university's Lyon Arboretum. The St. John Plant Science Laboratory building on the Mānoa campus, which houses the botany department, is named after him.
Not long after his arrival in Hawaii, he joined the Bernice P. Bishop Museum's Mangarevan Expedition of 1934, which returned with perhaps the richest collection of Polynesian plants ever made.