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 a mere 400, but has grown rapidly, especially in the latter half of the 20th century, led by the utilization of geothermal resources and development of woolens industry. The population was 5849 on 1. December 1999.
The area around the Athletic Center at Varmá is one of the country's best recreational areas. Emphasis is focused on catering to a variety of sports for children as well as adults.
The residential area extends from Leiruvogur into the nearby valleys, and curves around the mountains. It is close to beautiful nature, with an attractive and diverse community and cultural life, and offers numerous outdoor activities and a family-friendly environment.
Travelers who head out of Reykjavík towards the west and north of Iceland will pass through Mosfellsbær on route 1, while the road to the Þingvellir National Park, route 36, branches off in Mosfellsbær up the Mosfellsdalur valley.
The town is bordered by the ocean at Leiruvogur. Three rivers run to the sea: Leirvogsá, Kaldakvísl and Varmá
