Molenaar worked as a park ranger and mountain guide in Mount Rainier National Park, climbing the mountain over 50 times as a guide and on personal trips, via more than a dozen different routes including three first ascents. He participated in the 1946 second ascent of Mount Saint Elias in Alaska. He was a member of the Third American Karakoram Expedition, a 1953 mountaineering expedition to K2 in which the party became trapped during a severe storm. Along with Big Jim Jim Whittaker and Robert F. Kennedy, he was a member of the 1965 climb and first ascent of Mount Kennedy in the Yukon, named after John F. Kennedy.