Muir was a preservationist and naturalist. His activism helped to preserve the Yosemite Valley, Sequoia National Park and other wilderness areas. The Sierra Club, which he founded, is now one of the most important conservation organizations in the United States.

In early March 1867, Muir was injured while working as a sawyer in a factory that made wagon wheels: a tool he was using slipped and struck him in the eye. This accident changed the course of his life. He was confined to a darkened room for six weeks, worried whether he’d ever regain his sight. When he did, "he saw the world—and his purpose—in a new light," writes Marquis. Muir later wrote, "This affliction has driven me to the sweet fields. God has to nearly kill us sometimes, to teach us lessons." From that point on, he determined to "be true to myself" and follow his dream of exploration and study of plants.

A Thousand Mile Walk to the Gulf recounts Muir's walk of about 1,000 miles (1,600 km) from Indiana to Florida. He had no specific route chosen, except to go by the "wildest, leafiest, and least trodden way I could find." This journal is the earliest of Muir's writings and autobiographically bridges the period between The Story of my Boyhood and Youth and My First Summer in the Sierra.