Lassen Peak Reserve was not created until 1905, less than a month before Congress approved the Transfer Act, which took the reserves away from the General Land Office and the Department of Interior. Instead, the newly created U.S. Forest Service managed the lands under the Department of Agriculture. The reserves then became the National Forests. In 1908 Lassen absorbed portions of Diamond Mountain National Forest and exchanged lands with Plumas National Forest and Shasta National Forest.