Wildfire Prevention and Drought Mitigation Act of 2021 This bill categorically excludes forest management activities carried out on specified public lands from requirements to conduct environmental assessments and environmental impact statements. In order to be excluded from environmental review, the primary purposes of such activities must be to protect a municipal or tribal water source from damage caused by wildfire; improve watershed or habitat conditions; improve, maintain, or restore water yield or quality; improve, maintain, or restore snowpack; or adapt the forest landscape to an increased threat of drought. The bill limits the categorical exclusion to activities that manage up to 10,000 acres of land. However, activities that manage up to 30,000 acres of land may be categorically excluded from environmental review if the activities are located in areas that (1) are in a severe, extreme, or exceptional drought; or (2) have been in such droughts in the previous five years.