Clinton County was formed on February 19, 1810 from sections of Highland County and Warren County. It was named after George Clinton, a soldier, politician, governor of New York, and vice president of the United States. The entire county lies within the Little Miami River watershed, with the exception of extreme eastern areas, which are within the Scioto River watershed. Clinton County lies within the till plains physiographic region, and is somewhat flat or gently rolling. The northern two thirds of the county were covered with an ice sheet during the Wisconsinan Stage, while the southern third was covered by ice sheets during the much older Illinoian Stage. Because of such a history with ice, glacial features are readily found on the landscape, such as moraines and kames.
The general elevation of the county is roughly 1,050 feet (320 m) above sea level, and there are few areas in the county that deviate from this more than 200 feet (61 m).
The climate of Clinton County can be classified as humid continental. It is one of the coolest and wettest counties in southern Ohio, although differences between it and other southern Ohio counties are usually very slight. Clinton County averages 42 inches (1,100 mm) of precipitation per year, including 30 inches (760 mm) of snow (Note: 1 inch of snow does not equal one inch of precipitation). Average July high temperatures reach mid- and upper 80s (ºF), although temperatures above 90ºF are common; lows are typically mid-60s (ºF). January high temperatures typically reach the low 30s (ºF), while lows generally bottom out in the 10s (ºF), although lows in the single digits and even below 0ºF are common.
Interstate 71 crosses the northern third of the county, trending northeast to southwest. It connects Clinton County to Columbus, Ohio and Cincinnati. Clinton County is part of the Cincinnati-Middletown-Wilmington Combined Statistical Area, although there is little true urban activity in the county. U.S. Route 68 is the major north-south route through the county, while U.S. Route 22 runs east-west. Several other state and local highways serve the residents of Clinton County.