He was born in Allen, Michigan to Hiriam A. St. John and his wife Lois Bacon; the youngest of a family of four sons and two daughters.
He was awarded a John Tyndall Fellowship and studied for a year in Berlin before returning to earn his Ph.D. from Harvard in 1896.
After teaching for a year at the University of Michigan, he became an associate professor of physics at Oberlin College.
He also made observations of the planet Venus, showing that there was insufficient oxygen in the atmosphere to support life (as we know it).
He was elected to the United States National Academy of Sciences in 1924 and the American Philosophical Society in 1928.