The immediate need to treat forest fuels is often justified as a need to reduce potential fire behavior as well as improve or maintain forest health. Millions of hectares are at risk of unusually severe fires in the United States, and fuel treatments are being prescribed at unprecedented scales. In many cases, mechanical treatments with heavy equipment are the most efficient or economical method to reduce fuels. Despite the large-scale emphasis on both mechanical fuel modifications and forest health, few studies of fuel treatment effects have examined impacts to forest soils. We evaluated fuel treatment effects on soil compaction in a managed Sierra Nevada mixed-conifer forest using a fully replicated study design with three treatments: Thin, Thin + Burn, and an untreated Control. To examine impacts of mastication equipment that travels throughout a stand to reduce fuels, soil sampling was stratified to address effects at the scale of the treatment unit, the skid trail network, and the non-skid trail area. At all scales, the Thin and Thin + Burn did not increase soil bulk density compared to the Control. At the treatment unit level, soil strength was increased in the Thin + Burn relative to Control, but this was attributed to increased strength in skid trails rather than in the non-skid portion of the stand. The compacting forces of the masticator were buffered by the debris bed it created, and no significant compaction due to mastication was observed away from skid trails. Soil strength appeared to be a more sensitive measure of compaction, although a very weak relationship was observed between soil bulk density and soil strength. Despite frequent stand entries prior to these fuel treatments, the cumulative extent of detrimental compaction was not increased as a result of the Thin and Thin + Burn treatments. Mean soil strength in skid trails was consistently greater than in non-skid trail areas to a depth of nearly 60 cm. Measures to avoid the creation of new skid trails will help curtail increased soil compaction in managed forest stands, and particularly in fuel treatment areas that may require repeated entries to remain effective.