The MABR is situated within the traditional territories of seven First Nations communities and shares similar boundaries with the Regional District of Nanaimo. The biosphere region is approximately 1200 square kilometers. The vertical elevation spans just over 2100 meters from the highest peak on Mt. Arrowsmith (1817m) to 300 meters into the Salish Sea. This vertical range makes the MABR unique among Canadian Biospheres.

The geographical boundary is defined by five watersheds: Englishman River, Little Qualicum, French Creek, Nanoose Creek, and Bonnel Creek.

There are over 45,000 people living within the boundary of the MABR, most of whom live within five kilometers of the Salish Sea. The average age of citizens within the MABR is 60 years, which is the highest average age per capita in Canada.