[2] The county is named in honor of Colonel John Allen, who was killed leading his men at the Battle of Frenchtown during the War of 1812.

[a] Allen County comprises the Lima metropolitan statistical area, and as of the 2020 census, the population was 102,206.

Thus the area now comprising Allen County was off-limits to European settlement until the Treaty of Maumee Rapids in 1817.

The latter treaty opened the way for the Ohio Legislature on March 1, 1820, to create fourteen counties, including Allen,[6] which was defined as Ranges 5 through 8 east and Towns 3 through 6 south.

The first permanent settlement within Allen County's present day bounds took place in 1824, when Christopher S. Wood and his family settled in section 7 of Bath Township.

[citation needed] On February 12, 1829, an act of the legislature set aside land for a "county town".

[8] Wood was appointed commissioner to determine the location of this "seat of justice" for Allen County.

He laid out plots of land to be sold in section 31 of Bath Township,[9] and the plat was filed April 20, 1831, creating what was the beginning of the city of Lima.

Present at this meeting were Commissioners James Daniels, John G. Wood, and Samuel Stewart.

[citation needed] The meeting was held at the cabin of James Daniels, which was on the bank of the Ottawa River near the current location of Market Street.

[10] Patrick G. Goode of Montgomery County, special prosecuting attorney at that session, is given credit[citation needed] for having offered up the name of "Lima" (capital of Peru and source of the quinine used to treat the malaria prevalent in the area of the Great Black Swamp).

The county's western part gained a significant stretch of the Miami and Erie Canal, which was completed in 1845.

The reorganization also brought the towns of Spencerville, laid out in 1844 on the canal,[13] and Bluffton within the bounds of the county.

Run every August in Lima since 1851, the Fair is among Western Ohio's foremost agricultural showcasing events.

The fair attracted many nationally known performers during the 1980s and 1990s, and combined with significant renovations to the county show grounds, these factors led to a marked increase in attendance and notoriety over the past two decades.