In December 1911 he announced retirement from balloon racing and airplanes at the behest of his wife, father-in-law, and various business associates.

Harmon returned to flight in November 1913 while spending time in England, participating in a flight from London to Paris with Claude Grahame-White. In April 1915 he, along with several other aviators and wealthy friends, participated in a scheme to commute regularly from New York to points at the eastern end of Long Island by airplane.

After moving to Paris in the early 1920s, Harmon became an advocate for various aviation causes. In December 1924 he offered to replace the Gordon Bennett Cup after it had been won by Belgium; the following year he proposed a memorial in Paris to Lafayette Escadrille aviators; the model for the monument became the Harmon Trophy for aviation. In 1926 Harmon founded and financed the International League of Aviators (Ligue Internationale des Aviateurs), followed by a promotional tour of Europe financed by the King of Belgium and using a Caproni bomber on loan from the Italian Air Force.

In May 1927 Harmon was on hand for Charles Lindbergh's arrival in Paris; Harmon met Lindbergh during post-flight receptions. In June, Harmon installed a plaque commemorating Lindbergh at Le Bourget airport in Paris. Wreckage of Harmon's biplane at the Boston aviation meet In December 1928 Harmon sent a proposal to the League of Nations for an international air force that could be used by the League to prevent wars. While the proposal received some support, it was never seriously considered. In September 1930 Harmon made a flight in Villacoublay, France, commemorating his Long Island Sound crossing; with him on the flight was Maurice Farman, designer of the biplane Harmon used in the crossing.