Up until 2012 Njombe District was among the districts of the Iringa Region. In 2012, the area that had been within the old Njombe District was incorporated into the new Njombe Region. Within the new region there were created three districts that had wards that were formerly within Njombe District: Njombe Rural District, Njombe Urban District (Njombe Town Council) and Wanging'ombe District. In addition Njombe Region received Makete District and Ludewa District from Iringa Region.
The name Njombe originates from a name of a tree species "Mdzombe" that was by then dominant in the place called Mdandu which was a German "Boma" during German colonialism, this tree species has a medicinal value.
According to the 2002 Tanzania National Census, the population of the Njombe District was 420,348. A large population belong to the Christian religion and little Muslims and some Local Religion.
The main economic activities of the people of Njombe include cultivation of food crops. Njombe was one of the largest producers of Irish Potatoes, Maize and partly beans.
As for cash crops, timber and electric poles are produced at large amounts. It has come up of recent that a large population has embarked onto planting trees for timber. The common tree species grown here include Pines at a very large percent and a little of Eucalyptus and Cypress.
Timber is sold directly to Dar es Salaam where there is the larger market and the other is locally sold for carpentry, building and construction activities.
As of 2002[update], Njombe District was administratively divided into twenty-five wards:
- Njombe Mjini