Cook began practicing medicine after graduating from New York University in 1890. He soon achieved fame as an explorer, serving as surgeon on Peary's first Arctic expedition (1891–92) and leading others to explore and climb Mt. McKinley (1903–06). Cook's claim that he had reached the North Pole on an expedition in 1908 was immediately disputed by Peary. Cook's Eskimo companions on his journey later asserted that he had stopped short hundreds of miles south of the Pole, and that the photographs of his expedition were actually shot at locations far distant from the North Pole. The controversy between Cook and Peary lasted until World War I, after which time public support for Cook's claim disappeared. Cook was later charged with fraudulent use of the mails and imprisoned in 1923. He was paroled in 1930 and given a presidential pardon in 1940.