Part of Herkimer County was included in the Macomb's Purchase of 1791, during the wide-scale sale of public lands after the state forced Iroquois tribes allied with the British during the war to cede their territory.

Suddenly the state was selling 5 million acres (20,000 km2) of land in upstate, central and western New York.

By the mid-19th century, they had begun to specialize in dairy farming and created a cheese industry that supplied the New York City market, among others.

[5] Herkimer County is in central New York State, northwest of Albany, and east of Syracuse.

20.6% were of Italian, 16.3% German, 13.9% Irish, 9.3% English, 7.7% Polish, 6.2% American and 5.2% French ancestry according to Census 2000.

Since 2022, the entire county is within New York's 21st congressional district, presently held by Republican Elise Stefanik.

As of 2023[update] the county legislature is almost entirely Republican: the single Democratic legislator also ran on the Conservative Party line.

[14] In 2025, the chairman of the Herkimer County Legislature took a plea deal settling charges of drug possession, an Open-container law violation, and failing to safely store a firearm.

[16] Ilion was home to the original factory of the Remington Arms Company until it was closed in March of 2024.

Up to the latter 1950s, New York Central Railroad trains such as the North Shore Limited (New York-Chicago) made stops at Little Falls.

[20] Until 1965, the New York Central operated passenger trains through Thendara in the upper part of the county.