Based on the Köppen climate classification, Mount Dennis is located in a subarctic climate with cold, snowy winters, and mild summers. Temperatures can drop below −20 °C with wind chill factors below −30 °C. Weather conditions during winter make Mount Dennis one of the premier places in the Rockies for ice climbing. Precipitation runoff from Mount Dennis drains into tributaries of the Kicking Horse River which is a tributary of the Columbia River.