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.

In 1931 the Clifford B. Harmon Cup was created, to be awarded to amateur golfers.

Clifford Harmon continued to personally award the Harmon Trophy until 1938.

Later business ventures In 1912 Harmon was part of a syndicate that purchased Oklahoma oil leases from Charles Page for $1.25 million dollars. Also announced was a 2000 barrel-a-day refinery to be located at Sand Springs, Oklahoma. Harmon presenting the Harmon trophy in 1927 In 1915 Harmon became president of Mirror Films Incorporated, a film production company formed to apply business practices to film and "look upon film...as so much canned product". The company exhausted its working capital by mid-1916 and was subsequently mired in lawsuits (notably a breach of contract lawsuit filed by Nat Goodwin) through 1918 which put the company out of business.

Personal life Clifford Harmon married Zephyr Hord on September 18, 1887, in Dayton, Ohio; they divorced sometime after 1898. Harmon was later engaged to be married to Blanche Freeman, but broke the engagement in April 1904 to marry Louise Adele Benedict, daughter of Commodore Elias Cornelius (E.C.) Benedict. Freeman sued Harmon for breach of promise in 1909 (after initially suing for $100,000 and withdrawing the suit at Harmon's request), receiving an award of $15,000 in 1910. The award was set aside on appeal and the case retried later.