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County · code 39061

Hamilton CountyOhio

Hamilton County is a county in Ohio with its seat in Cincinnati, formed in 1790 by the Northwest Territory.

  • Founded1790
  • County seatCincinnati
  • Named forAlexander Hamilton
  • Land area405 sq mi + 7.2 sq mi water
  • Population830,639 2020 Census
  • Parcels mapped420,120 checked 15 October 2026
Open the free map Click any parcel in Hamilton County: acreage, parcel id, address and who publishes the map. No sign-in.

Updated

ButlerKenton, KYBoone, KYCampbell, KYClermontDearborn, INWarren Cincinnati Census outline · seat marked

Where it is

Hamilton on the map of Ohio

Ohio has 88 counties and county equivalents, each with its own records office. Shapes with a page are links.

Adams CountyAllen CountyAshland CountyAshtabula CountyAthens CountyAuglaize CountyBelmont CountyBrown CountyButler CountyCarroll CountyChampaign CountyClark CountyClermont CountyClinton CountyColumbiana CountyCoshocton CountyCrawford CountyCuyahoga CountyDarke CountyDefiance CountyDelaware CountyErie CountyFairfield CountyFayette CountyFranklin CountyFulton CountyGallia CountyGeauga CountyGreene CountyGuernsey CountyHancock CountyHardin CountyHarrison CountyHenry CountyHighland CountyHocking CountyHolmes CountyHuron CountyJackson CountyJefferson CountyKnox CountyLake CountyLawrence CountyLicking CountyLogan CountyLorain CountyLucas CountyMadison CountyMahoning CountyMarion CountyMedina CountyMeigs CountyMercer CountyMiami CountyMonroe CountyMontgomery CountyMorgan CountyMorrow CountyMuskingum CountyNoble CountyOttawa CountyPaulding CountyPerry CountyPickaway CountyPike CountyPortage CountyPreble CountyPutnam CountyRichland CountyRoss CountySandusky CountyScioto CountySeneca CountyShelby CountyStark CountySummit CountyTrumbull CountyTuscarawas CountyUnion CountyVan Wert CountyVinton CountyWarren CountyWashington CountyWayne CountyWilliams CountyWood CountyWyandot CountyHamilton County Hamilton
Counties of Ohio · Hamilton highlighted

History

Founded in 1790

An old deed may name a county that no longer exists, or a boundary that has since moved.

Hamilton County was created in 1790 from non-county area of the Northwest Territory and was named after Alexander Hamilton. Between 1796 and 1798, the county lost significant territory through the creation of Wayne, Adams, and Ross counties.

Boundary changes from the Newberry Library, Atlas of Historical County Boundaries; the 5 that matter most are shown first.

  1. 1790
    Hamilton County created Hamilton created by the Northwest Territory from non-county area. · Terr. Papers U.S., 3:294-295; Chase, 3:ch. 331/p. 2096
  2. 1792
    Gained territory Hamilton gained from Washington and from non-county area in the Northwest Territory; Hamilton included the eastern part of present Michigan.
    from Washington
    · Terr. Papers U.S., 3:363-364
  3. 1796
    Lost territory Hamilton lost to creation of Wayne (Northwest Territory, extinct); Hamilton eliminated from present Michigan.
    to Wayne
    · Terr. Papers U.S., 2:567-568 and 3:447; Chase, 3:ch. 332/p. 2096
  4. 1797
    Lost territory Hamilton lost to creation of Adams.
    to Adams
    · Terr. Papers U.S., 2:612-613; Chase, 3:ch. 333/p. 2096
  5. 1803
    Became part of Ohio Hamilton became a county in the state of Ohio. Hamilton gained from Knox (Indiana), exchanged with Clark (Indiana); Hamilton eliminated from present Indiana.
    from Knox (Ind.)from Clark (Ind.)
    · Ind. Terr., Exec. Journal, 114-115; Pence and Armstrong, 218; U.S. Stat., vol. 2, ch. 40 [1802], secs. 1-3/pp. 173-174; Van Zandt, 112
Show all 11 changes (6 more)
  1. 1790
    Hamilton County created Hamilton created by the Northwest Territory from non-county area. · Terr. Papers U.S., 3:294-295; Chase, 3:ch. 331/p. 2096
  2. 1792
    Gained territory Hamilton gained from Washington and from non-county area in the Northwest Territory; Hamilton included the eastern part of present Michigan.
    from Washington
    · Terr. Papers U.S., 3:363-364
  3. 1796
    Lost territory Hamilton lost to creation of Wayne (Northwest Territory, extinct); Hamilton eliminated from present Michigan.
    to Wayne
    · Terr. Papers U.S., 2:567-568 and 3:447; Chase, 3:ch. 332/p. 2096
  4. 1797
    Lost territory Hamilton lost to creation of Adams.
    to Adams
    · Terr. Papers U.S., 2:612-613; Chase, 3:ch. 333/p. 2096
  5. 1798
    Gained territory Hamilton gained from Knox (Indiana) in present Indiana and Ohio.
    from Knox (Ind.)
    · Terr. Papers U.S., 3:509; Chase, 3:ch. 335/p. 2097
  6. 1798
    Lost territory Hamilton lost to creation of Ross.
    to Ross
    · Terr. Papers U.S., 3:510; Chase, 3:ch. 336/p. 2097
  7. 1798
    Lost territory Hamilton lost to Adams.
    to Adams
    · Terr. Papers U.S., 3:510-511; Chase, 3:ch. 337/p. 2097
  8. 1800
    Lost territory Hamilton lost to creation of Clermont.
    to Clermont
    · Terr. Papers U.S., 3:526; Chase, 3:ch. 339/p. 2097
  9. 1803
    Became part of Ohio Hamilton became a county in the state of Ohio. Hamilton gained from Knox (Indiana), exchanged with Clark (Indiana); Hamilton eliminated from present Indiana.
    from Knox (Ind.)from Clark (Ind.)
    · Ind. Terr., Exec. Journal, 114-115; Pence and Armstrong, 218; U.S. Stat., vol. 2, ch. 40 [1802], secs. 1-3/pp. 173-174; Van Zandt, 112
  10. 1803
    Lost territory Hamilton lost to creation of Butler, Greene, Montgomery, and Warren.
    to Butlerto Greeneto Montgomeryto Warren
    · Ohio Laws 1803, 1st assy., ch. 4, secs. 1-4, 8/pp. 9-12; Chase, 3:ch. 345/pp. 2099-2100
  11. 1808
    Lost territory Hamilton lost to Butler.
    to Butler
    · Ohio Laws 1807, 6th assy., ch. 4/p. 10; Chase, 3:ch. 363/p. 2106

Land and parcels

What the maps cover in Hamilton

Who publishes each layer, how good it is, and when we last checked.

Official
from the office that keeps the record
Good enough
a reliable statewide or national copy
Fallback
a guide only; check the official record
Missing
nothing public found yet

People and land

Hamilton by the Census

Population
830,639 2020 Census count
Housing units
379,015 2020 Census count
Land area
405 square miles
Water area
7.2 square miles
People per sq mi
2,049 population ÷ land area

Source: US Census Bureau, 2020 Census; US Census Bureau, 2020 Gazetteer Files. Not endorsed or certified by the Census Bureau.

See your own lot on these maps

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Questions

Common questions about Hamilton County

When was Hamilton County founded?
Hamilton County was founded in 1790, and named after Alexander Hamilton.
What is the county seat of Hamilton County?
The county seat of Hamilton County is Cincinnati.
Is there an online parcel map for Hamilton County?
Yes. Hamilton County publishes a parcel map of 420,120 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.
Has Hamilton County's boundary or name changed?
Hamilton County was created in 1790. Since then the Atlas of Historical County Boundaries records 10 later changes, chiefly involving Adams County, Butler County and Knox (Ind.) County.
Is my lot in a flood zone?
We do not have flood maps for Hamilton County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

Nearby

Counties next door