The district is situated in a vast floodplain, between the Kilombero River in the south-east and the Udzungwa-Mountains in the north-west. On the other side of the Kilombero River, in the south-east, the floodplain is part of Ulanga District.
According to the last census in 2002, the population of Kilombero District is 321,611  The main ethnic groups are Wapogoro, Wandamba, Wabena, and Wambunga and several others in small proportions.
The area is predominantly rural with the semi-urban district headquarters Ifakara as major settlement.
The majority of the villagers are subsistence farmers of maize and rice. There are large plantations of teak wood in the Kilombero and the neighbouring Ulanga districts. In the north-west of the district, Illovo Sugar Company's sugar-cane plantations occupy most of the lowlying area.
For parliamentary elections, Tanzania is divided into constituencies. As of the 2010 elections Kilombrero District had one constituency:
- Kilombrero Constituency
- MSABI - Safe Water for Better Health Ifakara. Award winning water, sanitation and hygiene program[further explanation needed]
- Kilombero District (Approved) at GEOnet Names Server, United States National Geospatial-Intelligence Agency
- "2002 Population and Housing Census General Report: Morogoro: Kilombero".[dead link]
- "Morogoro Region Socio-Economic Profile" (pdf). The Planning Commission, Dar es Salaam, and The Regional Commissioner's Office, Morogoro. 1997. Archived from the original on 30 May 2004.
- Corti, Graham, Fanning, Eibleis, Hinde, Roy, and Jenkins, Richard, 1999. A Conservation Assessment the Amphibian Communities of Miombo Woodland, Evergreen Forest and a Teak (I'ectona Grandis) Plantation in the Ulanga Valley, Tanzania (Technical Report 3). Frontier-Tanzania Savanna Research Programme, Dar es Salaaml
- Hinde, R., Corti, G., Fanning, E., & Jenkins, R., 1999a, A conservation assessment of large mammal use of miombo woodland, evergreen forest and a teak plantation in the Ulanga Valley, Tanzania, Frontier Tanzania Savanna Research Programme, Technical Report No. 2., unpublished, Frontier Tanzania, Dar es Salaam, Tanzania.
- Jenkins R., Roettcher, K, Corti, G & Fanning, E, 2000a, Large mammals and teak plantations in the Ulanga Valley, Annual Report, unpublished, Frontier Tanzania, Dar es Salaam, Tanzania.
- "Organisations located in Kilombrero District - Tanzania". African Development Information.