Essex was one of the first counties to be settled in Upper Canada, later to become Ontario, mostly by French people in the mid-18th century. Around 1749, the first permanent settlements began to appear on what is now the Canadian side of the Detroit River.
In the late 18th Century and early 19th century the French ventured east along the south shore of Lake Saint Clair and settled in the present-day areas of Belle River (Belle-Rivière), Rochester, Tecumseh, Saint-Joachim and Stoney Point (Pointe-aux-Roches). These communities still have a large francophone population.
In 1791, the province of Upper Canada was formed. In 1792, Upper Canada was divided into 19 counties, of which Essex was the 18th and part of the Western District. At that time, the eastern boundary of Essex County extended further east into what is now Kent County.
The establishment of good roads led to further settlement in the area of what is now Leamington. Settlers of this era were often emigrants from Britain and Ireland; in the 1840s the Potato Famine led to significant immigration.
Essex County was also a destination of the Underground Railroad by which African slaves in the 19th-century United States escaped to freedom.
By the late 19th century Essex County had seen fur trading and logging, land clearing and farming, road building and railway development, saw mills and gristmills, railway stations and water ports. By this time the forests were disappearing, replaced by fertile farmland that Essex County is still known for today.