In 1917, President Woodrow Wilson asked him to run the nation's Federal Fuel Administration during World War I. Garfield's duty was to conserve the coal supply and keep the price within reasonable bounds. Local committees were appointed throughout the country to study local conditions and their reports formed the basis for the prices fixed in different localities. The ensuing winter was unusually severe, and serious shortage of coal threatened. Because of the shortage of coal in the northeastern United States, especially in New York City and Ohio, Garfield's administration of the office was severely criticized in the press, but an investigation by the United States Congress showed that the shortage was due to failure of the railroads to meet the extra demands upon them, and federal control of the roads was instituted on December 28, 1917. Garfield also issued his “idle Mondays” order in January 1918, which closed non-essential industries for five consecutive days beginning January 18 and on every Monday thereafter up to March 25. This roused a storm of protest from many manufacturers, and the U.S. Senate voted a resolution, requesting postponement, but this reached him after the order had been signed. On February 14, however, the order was suspended and priority for certain shipments substituted. He disapproved of the method of settling the coal strike in December 1919 and resigned his office, resuming that of president of Williams College.
He married his second cousin, Belle Hartford Mason, and had four children by her. He is buried in the faculty cemetery at Williams College.