They established the towns of Losantiville (later Cincinnati), North Bend, and Columbia.

Residents named the county in honor of Alexander Hamilton, who was the first Secretary of the Treasury of the United States and a founder of the Federalist Party.

The United States forcibly removed most of the Shawnee and other Indian peoples to move to locations west of the Mississippi River in the 1820s.

Rapid growth occurred during the 1830s and 1840s as the area attracted many German and Irish immigrants, especially after the Great Famine in Ireland and the revolutions in Germany in 1848.

During the Civil War, Morgan's Raid (a Confederate cavalry campaign from Kentucky) passed through the northern part of the county during the summer of 1863.

The Sharonville Engineer Depot was constructed by the United States Army in northern Hamilton County in 1942, and continued to be used by the General Services Administration and then the Defense Logistics Agency after 1949.

[6] The county lies in a region of gentle hills formed by the slopes of the Ohio River valley and its tributaries.

[8] The highest land elevation in Hamilton County is the Rumpke Sanitary Landfill at 1,045 feet (319 m) above sea level in Colerain Township.

[20] The Cincinnati Metropolitan Statistical Area, over the last three decades has seen a 19 percent increase in population.

[20] As of 2020, the members of the Hamilton Board of County Commissioners are Denise Driehaus, Stephanie Summerow Dumas, and Alicia Reece.

[22] Other elected officers include Dusty Rhodes (Auditor), Joe Deters (Prosecutor), Charmaine McGuffey (Sheriff), Eric Beck (Engineer), Scott Crowley (Recorder), Jill Schiller (Treasurer), and Lakshmi Sammarco (Coroner).

It long favored Republican candidates in national elections, but has trended Democratic in recent years.

In fact, it was one of the few counties in Ohio to swing toward the Democrats in 2016 even as the state as a whole swung toward the Republicans.

Richard Cordray in his failed 2018 bid was the first Democrat to win the county in a gubernatorial election since Dick Celeste in 1982, and only the second since Michael DiSalle in 1958.

[26] In the 2006 Ohio elections, both Ted Strickland and Sherrod Brown lost the county by less than 2,000 votes while winning statewide by 24 and 12 points, respectively.

[27] Democrats had previously regained majority control of the Board of Commissioners in 2016 with the election of Denise Driehaus.

In 2019, longtime Democratic Commissioner Todd Portune announced his resignation from the Board due to health problems.

Portune's Chief of Staff, Victoria Parks, was appointed to serve the remainder of his term (through the November 2020 general election).

With Parks' appointment, the Board of Commissioners became for the first time all-female and majority Black.

In 2012, The Washington Post named Hamilton as one of the seven most important counties in the country for that year's election.

[34] The county also has a vocational school district, the Great Oaks Institute of Technology and Career Development.

Major sports teams are listed under the communities in which they are located, primarily Cincinnati.

It does not include townships that became part of Butler, Warren, Clermont, Montgomery, and other counties.