Marshall was drawn to the outdoors. He discovered his passion for exploring, charting, and a love of climbing mountains, in part through the writings of Verplanck Colvin, who during the post-Civil War decade surveyed the woods of northern New York. Throughout his life, Marshall kept a series of hiking notebooks, which he illustrated with photographs and filled with statistics. In 1915, Marshall climbed his first Adirondack peak, the Ampersand Mountain, alongside his brother George and family friend Herb Clark, a Saranac Lake guide. The two brothers learned the arts of woodcraft and boating through Clark, who accompanied them on most of their longer trips during adolescence and early adulthood. By 1921, they became the first climbers to scale all 42 Adirondack Mountains believed to exceed , some of which had never been climbed. In 1924, the three became the first Adirondack Forty-Sixers, hikers who have climbed to the summits of all 46 High Peaks of the Adirondacks.