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.

Following the successful commissioning of the Trout Lake plant, in 1911 Greenway was recruited by the Calumet and Arizona Mining Company (led by former US Steel executives, the combined entity created by J.P. Morgan which included Carnegie Steel) to develop their newly acquired New Cornelia Mine in Ajo, Arizona.

He also served one year as a regent of the University of Arizona before the United States entered World War I.

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.

"[7]Greenway was returned to active service as a lieutenant colonel at the dawn of America entering World War I.

Originally based at Toul Sector, he partook in the Battle of Cantigny, the first large-scale counterattack on German lines by the American Expeditionary Forces (AEF) with the 1st Battalion of the 26th Infantry, 1st Division, commanded by Major Theodore Roosevelt Jr., the son of his commander during the Spanish–American War, Theodore Roosevelt.

A Charles Henry Niehaus statue of Greenway's great-great-grandfather, Dr. Ephraim McDowell, was placed in the National Statuary Hall in 1929 by Kentucky, making them the only direct relatives to have shared the honor.