[3] The county was named after an early Army fortification, Fort Defiance, which was so named by Mad Anthony Wayne to signify the settlers' "defiance" of the Indians.

[6] As of the census[11] of 2000, there were 39,500 people, 15,138 households, and 11,020 families living in the county.

23.00% of all households were made up of individuals, and 9.50% had someone living alone who was 65 years of age or older.

As of the 2010 United States Census, there were 39,037 people, 15,268 households, and 10,792 families living in the county.

[12] In terms of ancestry, 42.3% were German, 12.5% were Irish, 9.6% were English, and 7.7% were American.

[15] Prior to 1940, Defiance County was a Democratic stronghold, voting Republicans only three times since 1856.

School districts include:[17] The closeness of elections in Defiance County has also been referenced in fiction; the ABC political drama Scandal in its second season had as the center of the ongoing plot of the first half of that season, a vote manipulation conspiracy which bent the presidential election towards Republican candidate Fitzgerald Grant based on tampering of the voting machines in Defiance County.