The county entity was effected in 2013, replacing Nairobi City Council, which had been the long-standing unit of local administration since before Kenya's independence.

The city county consists of eleven gazetted sub-counties and eighty-five electoral wards.

The Members of the County Assembly (MCAs) are elected from single-member districts and represent their respective electoral wards.

On 25 August 2022, after the general elections in Kenya, Johnson Sakaja was sworn-in as the fourth Governor of Nairobi County.

[6][5] The population density was highest in the sub-county of Mathare with 68,942/km2, followed in a distant second by Kamukunji with 25,455/km2, then Makadara with 16,150/km2, Kibra and Dagoretti with 15,311/km2, and 14,908/km2 respectively.