The city contains the site of Fort Defiance, built by General "Mad" Anthony Wayne in August 1794, during the Northwest Indian War, at the confluence of the Auglaize and Maumee rivers.
Fort Winchester was built on the same spot during the War of 1812, but it was a larger fort that extended southward somewhat along the Auglaize River.
Historical plaques in the sidewalks mark the full extent of Fort Winchester. In 1845 it was made the county seat of the newly created county, and in 1881 it became a city.
According to the Midwest Climate Center, Defiance's average January temperatures are a high of 31.2 °F (−0.4 °C) and a low of 17.1 °F (−8.3 °C).
Average July temperatures are a high of 84.2 °F (29.0 °C) and a low of 63.3 °F (17.4 °C).
There are an average of 16.4 days with highs of 90 °F (32 °C) or higher and an average of 138.8 days with lows of 32 °F (0 °C) or lower.
The record high temperature was 107 °F (42 °C) on June 28, 2012.Average annual precipitation is 35.60 inches (90.4 cm) with an average of 129.0 days with measurable precipitation.The wettest year was 1990 with 48.63 inches (123.5 cm) and the driest year was 1930 with 19.67 inches (50.0 cm).Normally, July is the wettest month with an average of 3.89 inches (9.9 cm) and February is the driest month with an average of 1.84 inches (4.7 cm).The most precipitation in one month was 12.00 inches (30.5 cm) in June 1981.The most precipitation in 24 hours was 4.63 inches (11.8 cm) on September 18, 1907.