According to the U.S. Census Bureau, the county has a total area of 20,955 km² (8,091 mi²). 20,488 km² (7,910 mi²) of it is land and 467 km² (180 mi²) of it is water. The total area is 2.23% water.

The Colorado River forms the county’s southeastern boundary, with Hoover Dam forming Lake Mead along much of its length. Las Vegas is frequently, yet incorrectly labeled a valley. By definition, Greater Las Vegas is a land basin or bowl, surrounded by four mountain ranges, with nearby Mount Charleston being the highest elevation at 11,918 ft, located to the northwest. Other than the forests on Mount Charleston, the geography in Clark County is a desert. Creosote bushes are the main native vegetation, and the mountains are mostly rocky with little vegetation.

Adjacent Counties
Lincoln County, Nevada - north
Nye County, Nevada - west
Inyo County, California - southwest
San Bernardino County, California - south
Mohave County, Arizona - east