Johnson attended graduate school for a short period at the University of Michigan before leaving for a job – one that would change his career trajectory. He started his career with the National Park Service, one that would span decades, at Yellowstone National Park in 1987. He has spent the last 25 years serving at Yosemite National Park, where he stumbled upon a photograph from 1899 of five black US Army infantry soldiers. This finding led him to research to learn more about Buffalo Soldiers, who were the authentic and original stewards of the parks system before the creation of the NPS. Since this discovery, he has dedicated his career to educating others about the role of Buffalo Soldiers and the parks, and by extension, the innate connection African Americans have to the environment, despite many suggesting otherwise. This experience would also profoundly impact his writing, connecting his educational training with his professional career. In 2011, Johnson published Gloryland, a novel that follows a sharecropper's son in becoming a Buffalo Soldier. Further harnessing his literary skills, Johnson started a narrative podcast, "A Buffalo Soldier Speaks," and wrote and performed Yosemite Through the Eyes of a Buffalo Soldier, 1904, a living history performance. Underlying every action with the NPS, he seeks to advance diversity within the parks and dismantle barriers to the parks.

Johnson is among the most prominent National Park Service Park Rangers due to his dedication to advancing park diversity. He appeared in Ken Burns' PBS series The National Parks: America's Best Idea, discussing topics ranging from bison to democracy and diversity within the national parks. Johnson has earned several awards for his work, including the 2013 Legacy Leadership Award from The African American Experience Fund of the National Park Foundation, the 2015 Superior Service Award from the US Department of the Interior, the 2021 Lifetime Achievement Award from Outdoor Afro, and the 2022 American Park Experience Award from the National Park Trust, among several others.