Harmon was a prominent Democrat, serving as an alderman of the city of Detroit in 1847 and two years as mayor, in 1852 and 1853,[1] as well as representing Michigan on the 1848 Democratic National Committee.[4] He was also a member of the state militia, being appointed a Colonel in 1850.[5] Following his stint as mayor, he was appointed by President Franklin Pierce as Collector for the Port of Detroit, where he served until 1857.[1] After he left the office of Collector, Harmon spent much of his time in Washington, D.C., during congressional sessions.[6]
During this time Harmon, as one biographer put it, "became a victim to the bad habits created by the customs of hospitality then existing in Detroit."[6] Through generosity to his friends, his fortune declined, and Harmon spent some time again working for the Free Press.[7] However, he later "conquered [his] habit" and became a total abstainer,[7] a "silent but eloquent example of a permanently reformed man."[6] John H. Harmon died August 6, 1888, in a hotel in Detroit.[8]