John Campbell Greenway was born in Huntsville, Alabama, to Dr. Gilbert C. and Alice White Greenway. On both sides, he was a direct descendant of a line of notable Americans dating to before, and during, the American Revolutionary War including William Campbell, Isaac Shelby, Samuel McDowell, Ephraim McDowell, and Addison White. He attended Phillips Academy, Andover followed by the University of Virginia for his undergraduate degree before earning a PhB in 1895 from Yale University. He was a member of the Book and Snakesecret society, president of his class, and a member of noted the Yale Football teams from 1892–1895 that went a combined 52–1–2 and were national champions four years in a row. Immediately following graduation, he joined the Carnegie Steel Company where he worked briefly before being commissioned in the 1st Volunteer Cavalry of the U.S. Volunteers at the outset of the Spanish–American War. After being removed from active duty at the end of the Spanish–American War in 1899, Greenway returned to steel and mining and held executive positions in a number of mine, steel, and railroad companies. He supervised development of United States Steel's open pit Canisteo Mine and Trout Lake Washing Plant in Coleraine, Minnesota, one of the first large-scale iron ore benefication plants in the world. Following the successful commissioning of the Trout Lake plant, in 1911 Greenway was recruited by the Calumet and Arizona Mining Company to develop their newly acquired New Cornelia Mine in Ajo, Arizona. He developed the Ajo townsite and developed the New Cornelia into the first large open pit copper mine in Arizona. He also served one year as a regent of the University of Arizona before the United States entered World War I.
Military career
Spanish–American War
Greenway volunteered for service in 1898 and joined Roosevelt's Rough Riders in the Spanish–American War. Originally commissioned a second lieutenant, he was then promoted to brevet then acting captain in the field by Colonel Roosevelt. Greenway earned a Silver Star for his courageous service at the Battle of San Juan Hill. He is referenced on numerous occasions by Roosevelt in his book The Rough Riders and a book of Greenway's own correspondence was turned into a book entitled It Was the Grandest Sight I Ever Saw: Experiences of a Rough Rider As Recorded in the Letters of Lieutenant John Campbell Greenway. In his book, "The Rough Riders", Roosevelt said about Lieutenant Greenway:
"A strapping fellow, entirely fearless, modest and quiet, with the ability to take care of the men under him so as to bring them to the highest point of soldierly perfection, to be counted upon with absolute certainty in every emergency; not only doing his duty, but always on the watch to find some new duty which he could construe to be his, ready to respond with eagerness to the slightest suggestion of doing something, whether it was dangerous or merely difficult and laborious."
