Shortly after graduation, Heintzleman joined the United States Forest Service and worked in Oregon and Washington. With the United States entry into World War I, he was transferred to Alaska to oversee lumber production. There, from 1918 till 1934, he served as Assistant Regional Forester in Ketchikan, Alaska. In this role he assisted the development of the region's pulp and lumber industries and was key to the construction of two pulp mills in southeast Alaska. During this time he wrote the 1921 Forest Service bulletin "The Forests of Alaska".