Ada County was created by the Idaho Territorial Legislature in 1864, partitioned from Boise County. It is named for Ada Riggs, the daughter of H.C. Riggs, a member of the legislature; he established the county and was a co-founder of Boise. Canyon County, which originally included Payette County and most of Gem County. As of the 2010 United States Census, the county had a population of 392,365, making it the state's most populous county, with 23.3% of the state's population.

The county seat and largest city is Boise, which is also the state capital.