The town was 'sited' on July 23, 1913, by Willis T. McConnell and named for two bachelor brothers, George and H.H. Harmon. The town went into decline during the Depression in the 1930s and virtually disappeared. The original cemetery is still in use. On January 15, 1951, two Northern Pacific freight trains collided outside of town.[5] The town has repopulated in the 20th century.[5]
Harmon's population was 259 as of the 2020 census,[6] the year in which it became a CDP.[7]