Killarney is a vast and beautiful tract of wilderness, with crystal-blue lakes, a spectacular range of white quartzite ridges, pine and hardwood forests, and boggy lowlands. Its beauty and magnificent, unspoiled scenery has inspired many artists, including members of the Group of Seven.

The park's most striking feature is the series of rocky ridges, mostly consisting of white quartzite, that dominate the landscape. These large, rounded hills are the remains of the La Cloche range, a ring of mountains that formed in Precambrian time, about 2 billion years ago, and towered higher than the present-day Rockies.

Four ice ages, over the last million years, scraped away the tops of the mountains, filled in some of the crevices between the bedrock outcrops, and left innumerable sandy ridges marking the temporary shorelines of ancient meltwater lakes. Along these former shorelines, archeologists have uncovered evidence of three prehistoric encampments, from 9000, 6500 and 1500 years ago.

The park is in the transition zone between the northern boreal forest region and the St. Lawrence-Great Lake lowlands, so there is a wide variety of plant and animal life within its boundaries. Some species, like black cherry and American beech, are rarely found this far north. More than 100 species of bird breed in the park.