The irregularly shaped county is bounded on the northeast by Gillespie County, on the east by Kendall County, on the south by Bandera County, on the southwest by Real County, on the west by Edwards County, and on the northwest by Kimble County. Kerrville is the county seat, and Ingram is the only other incorporated community. Kerr County is drained by the Guadalupe River and its tributaries and covers 1,107 square miles of undulating to hilly land with elevations that range from 1,500 to 2,000 feet above sea level.
Annual rainfall is thirty inches.
The county has a growing season of 216 days, and between 1 and 10 percent of the land is considered prime farmland. The county is in the Edwards Plateau vegetation area, characterized by buffalograss, wildrye, and switchgrass, and by live oak, shinnery oak, junipers, and mesquite trees. Kerr County is in a region that has been the site of human habitation for thousands of years. Archeological artifacts found in the area, particularly along the Guadalupe River and its forks, suggest that human inhabitants arrived between 6,000 and 10,000 years ago. During historical times Lipan Apaches, Comanches, and Kiowas hunted in the region.