Mills led the fight to preserve the area around Longs Peak as a national park, and used his speeches, his writing, and photography to lobby for the park. He wanted a park of about 1,000 square miles that would cover the area from Wyoming to Pikes Peak. Mills was aided by the Sierra Club, Daughters of the American Revolution, American Civic Association, the General Federation of Women's Clubs and especially, Freelan Oscar Stanley, founder of the Stanley Hotel. President Woodrow Wilson signed into law the bill that made the Rocky Mountain National Park the tenth national park on January 26, 1915. It was 352.5 square miles. He was called the Father of Rocky Mountain National Park by the Denver Post.