The natural landscape of Mosfellsbær is rugged, and 80% of the district lies more than 100 metres above sea level. Hills such as Mosfell, Grímannsfell, Helgafell and Úlfarsfell, all 200–300 metres high, dominate the landscape; their slopes are grassy with screes. Two valleys lie between the hills: Mosfellsdalur and Reykjadalur. Most of the district's population live in the latter valley. In 1900, the population of the Mosfellsbær district (then Mosfellssveit) was just 400, but has grown rapidly, especially in the latter half of the 20th century, led by the utilisation of geothermal resources and development of a woollen industry. The population was 5849 on 1 December 1999.