As a Yale graduate student, he accompanied Lieutenant Colonel George Armstrong Custer's 1874 Black Hills expedition as a naturalist.

)[3] In 1875, Colonel William Ludlow, who had been part of Custer's gold exploration effort, invited Grinnell to serve as naturalist and mineralogist on an expedition to Montana and the newly established Yellowstone Park.

Grinnell prepared an attachment to the expedition's report, in which he documented the poaching of buffalo, deer, elk and antelope for hides.

"[3]: 102  His experience in Yellowstone led Grinnell to write the first of many magazine articles dealing with conservation, the protection of the buffalo, and the American West.

He was also a member of the Edward Henry Harriman expedition of 1899, a two-month survey of the Alaskan coast by an elite group of scientists and artists.

From 1880 to 1911, he served as editor and president of the weekly Forest and Stream, and wrote articles and lobbied for congressional support for the endangered American buffalo.

In 1887, Grinnell was a founding member, with Theodore Roosevelt, of the Boone and Crockett Club, dedicated to the restoration of America's wildlands.

His principal translator (and also an informant) for both books was George Bent, a Cheyenne of mixed race who had fought for the Confederacy during the Civil War.

"[7] Selected papers by Grinnell were edited and published in 1972 by J. F. Reiger, a professor of history at Ohio University-Chillicothe and the former executive director of the Connecticut Audubon Society.