That early education did not seem to have amounted to anything when in 1883 Granjean set off for the United States to make a living from mining and trapping. He was in Nebraska for a short time before moving to the Wood River mining district in Idaho. In 1896 the gold of Alaska called. He spent three years searching for a strike, but ended up with little to show for it.
Grandjean moved back to Idaho and began mining near the headwaters of the Salmon River. He did well enough at that, but in 1906 he applied for a job as a ranger with the newly formed Forest Service. He took charge of the Sawtooth and Payette National Forests, then became the first supervisor of the Boise National Forest which was created from portions of the Sawtooth and Payette. His early training in forest management made him unusually qualified, since it was a new field in the U.S.
Grazers were not wild about the idea of the Forest Service telling them what to do, but Grandjean was a likable man who was able to eventually cool tempers. He was good at the often difficult job of representing the government, and stayed on as supervisor until 1922.
There is a valley, a creek, a mountain, and a campground named after Emile Grandjean.
There is also a living monument, in a sense. In 1912 Grandjean received some sequoia seedlings from John Muir. He planted two at his home in Boise. Two others were planted at the home of Fred and Alice Pittenger, both doctors. A later owner of the Grandjean home cut down the sequoias there, and only one survived at the Pittenger home. Eventually St. Lukes became the owner of that property and of the tree, which had become a Boise favorite. In June, 2017, the 98-foot-tall tree was moved across the street to make way for development at St. Lukes. It seems to be doing well in its new home near the Boise Little Theater.
Grandjean passed away in 1942 and is buried in the Canyon Hill Cemetery, Caldwell.