[1] Denman Island was first inhabited by Indigenous peoples including the Pentlatch and Sliammon as evidenced by middens, gravesites, and oral history.

In 1862 smallpox epidemic severely reduced its population and the survivors moved to join the K'ómoks people in nearby Comox.

[2] The Island was seen and mapped by Europeans during the 1791 voyage of the Spanish ship Santa Saturnina, under Juan Carrasco and José María Narváez.

Some areas have shallow soils over sedimentary rock and are mapped as Haslam shaly loam or Rough Stony-Haslam complex.

The eastern part has a loamy sand (Kye-Qualicum complex or Bowser series) on which arbutus is frequently present near the shore.