As an Interpretive Ranger at Yosemite National Park, Shelton is currently researching and interpreting the African American military history within Yosemite National Park. Specifically, Johnson is focusing on the role of the Buffalo Soldiers, the 9th Cavalry and the 24th Mounted Infantry, in protecting the national parks. It is a "work in progress" as he uncovers the layers of history since 1864, when President Lincoln signed the Yosemite Grant. He wrote and performs a living history performance called Yosemite Through the Eyes of a Buffalo Soldier, 1904, which is presented as an interpretive program at the park and at locations around the country.
Shelton began his career in Yellowstone National Park in 1987; but as of 2015, he has worked in Yosemite for 22 years of his 28-year career!
In 2009, he appeared in the Ken Burns documentary film The National Parks: America's Best Idea, and was called the "unexpected star" of the mini-series. Johnson attended a preview of the film at the White House that day, where he discussed the documentary with President Barack Obama. For his work with Ken Burns and Dayton Duncan on this film, he recieved the National Freeman Tilden Award as the best interpreting ranger in the National park Service!