The first fifteen years of National Park Service management of Sequoia and General Grant national parks witnessed significant human impacts on the environments of the two parks. Before 1916, for twenty-five years, the parks had been run mostly as strict preserves, not necessarily because of a strong philosophical commitment to preservation, but largely because pressure for tourist development and congressional appropriations for visitor facilities were limited. All this changed with the successful establishment of the Mather-Albright program in the years immediately following the First World War.

Determined to build and maintain public support for the national parks and their new National Park Service, Mather and Albright found successful ways to publicize the parks and succeeded in obtaining increasing amounts of development money. The result was a nearly logarithmic increase in parks' visitation and an equal increase in visitor impacts on natural resources. When combined with a visually based philosophy of national park management, which allowed almost no consideration of what little was known then of ecology, the unsurprising outcome was a significant setback in the biological health of the two parks.

The damage done in the 1920s unevenly affected the parks and their resources. In Giant Forest, and to a lesser extent in General Grant, the sheer physical impact of heavy visitation was unmistakable. In these geographically limited areas nearly all resources suffered. Unrestricted heavy camping trampled and destroyed forest undergrowth and meadow vegetation, while animals suffered habitat disruption and predator destruction. Most severely affected were large animals like bears, which were strongly attracted to human food sources, and mountain lions, which were regularly killed throughout the decade. Smaller species suffered too, however, and never again would it be possible to describe Giant Forest's fauna as natural or undisturbed.

National Park Service impacts were not limited to areas of heavy development, however. The resumption of cattle grazing in Sequoia was a setback that undid a quarter century of military protection, as did the increasing effectiveness of National Park Service fire suppression activities. Serious fire suppression support from Congress arrived in the two parks in the late 1920s just as three decades of fire suppression began finally to allow the dangerous accumulation of fuels. This coincidence would bear increasingly significant results as the next several decades passed.

In the surrounding Sierra and Sequoia national forests, the same period saw much less change in the state of the natural environment, for the Forest Service during these years did little more than continue the policies defined during its first decade of existence. Grazing remained widespread throughout the forests, and limited development in the form of roadbuilding, logging, and recreational settlement took place along the accessible western fringes of the forests. The Forest Service shared many resource management programs with the Park Service during these years, including fire suppression and predator control, but the immense size of the national forests, and their relative inaccessibility rendered them less susceptible to quick change. In many ways the wilderness resources of the national forests of the southern Sierra were protected better than those of the national parks of the same region during the 1920s. In 1926 Sequoia National Park doubled in size by absorbing the national forest lands at the headwaters of the North Fork of the Kern River. These lands were little different than they had been in 1910, shortly after the Forest Service had finally succeeded in controlling unpermitted sheep grazing. In comparison, many national park resources had suffered significantly since 1916. (The Forest Service had its plans, though. In the extensive high country of the Kings River drainage, the Forest Service, true to its mission, allowed studies to proceed that were intended to eventually lead to massive hydroelectric power development.) As the decade ended, though, the national forest wilderness of the southern Sierra remained almost completely intact, except for the continuing impacts of grazing.

In the last years of the 1920s, Superintendent White, whose understanding of his parks had grown considerably, began to realize what the National Park Service programs had wrought. He and others were beginning to note human impacts on the landscape and even on ecosystems, and were pondering the implications. Two more major waves of development would have to be weathered, however, before resource concerns would come to dominate National Park Service management of its southern Sierra national parks.