The phrase is also the Maasai name of the Nairobi river, which in turn lent its name to the city. However, it is popularly known as the "Green City in the Sun", and is surrounded by several expanding villa suburbs.Inhabitants of Nairobi are known as Nairobians, and the city is governed by the County Government of Nairobi, whose current governor is Evans Kidero.

Nairobi was founded in 1899 by the colonial authorities in British East Africa, as a rail depot on the Uganda Railway. The town quickly grew to replace Machakos as the capital of the colony in 1907. After independence in 1963, Nairobi became the capital of the Republic of Kenya. During Kenya's colonial period, the city became a centre for the colony's coffee, tea and sisal industry.

The city lies on the Nairobi River in the southern part of country, and has an elevation of 1,795 metres (5,889 ft) above sea level.

With a population of about 3.36 million estimated in 2011, Nairobi is the second-largest city by population in the African Great Lakes region after Dar es Salaam, Tanzania.

 According to the 2009 census, in the administrative area of Nairobi, 3,138,295 inhabitants lived within 696 km2 (269 sq mi). Nairobi is the 14th-largest city in Africa, including the population of its suburbs.

Nairobi is one of the most prominent cities in Africa, both politically and financially. Home to thousands of Kenyan businesses and over 100 major international companies and organisations, including the United Nations Environment Programme (UNEP) and the main coordinating and headquarters for the UN in Africa and Middle East, the United Nations Office at Nairobi (UNON), Nairobi is an established hub for business and culture.

 The Nairobi Securities Exchange (NSE) is one of the largest in Africa and the second-oldest exchange on the continent. It is Africa's fourth-largest exchange in terms of trading volume, capable of making 10 million trades a day. The Globalization and World Cities Study Group and Network (GaWC) defines Nairobi as a prominent social centre.