During and after the colonial period, French, British and American fur traders established relations with the historic peoples of the time.

The county was barely inhabited by European Americans until the 1830s, but this period was one of steady migration by settlers from New York and New England.

Toward the end of the 19th century, during the Great Depression, and the post–World War II baby boom, this area had periods of greater population increase.

Tiffin developed on both sides of the river, which flows north to its mouth at Lake Erie.

[7] The Great Lakes moderate temperatures somewhat, but Seneca County has essentially a continental climate.

Besides the fringe of the Great Black Swamp in the northwest, there was also an extensive area of marshland in the Bloomville area as well as smaller patches of swamp terrain which were formed due to the county's essentially level terrain.

[7] Native American inhabitants and later settlers used the region mainly for hunting fur animals, with little agriculture of note until the early 19th century.

This led to massive alteration of much of the local wildlife, with grassland and farmland animals replacing the native woodland fauna.

Migrant waterbirds, in ancient times commonly encountered throughout the region as they foraged in the swamps on their way south, are nowadays rare and concentrate on the few remaining waterbodies large enough to sustain them.

The passenger pigeon (Ectopistes migratorius) had several roosting (and probably nesting) places in the county when it was still wooded.

Removal of the forest had driven the birds away by the 1860s, foreshadowing its eventual total extinction due to large-scale logging which rendered this species unable to sustain the massive hunting pressure.

[7] Landbirds were apparently less seriously affected; apart from the passenger pigeon, the ruffed grouse (Bonasa umbellus), wild turkey (Meleagris gallopavo), golden eagle (Aquila chrysaetos) and marsh wren (Cistothorus palustris) had essentially or completely disappeared by 1900.

However, it is not known how many of the numerous species of New World warblers, most of which today only occur only as transient migrants, formerly bred in Seneca County.

[7] The Eskimo curlew (Numenius borealis), possibly extinct today, occurred as a transient in Ohio until about 1900; to what extent it migrated through Seneca County is not well known but even if it did it is unlikely that it was often seen after deforestation had gotten underway in earnest.

The rare Kirtland's warbler (Setophaga kirtlandii) is again increasing in numbers and may occasionally range as far north as Seneca County.

The ring-necked pheasant (Phasianus colchicus), another species introduced from Europe, never seems to have become really plentiful, though it has been a breeding resident since at least 1901.