Wildland Fire Mitigation and Management Commission Act of 2021 This bill directs the Departments of the Interior and Agriculture and the Federal Emergency Management Agency (FEMA) to jointly establish a commission to study and make recommendations to improve federal policies relating to wildland fire prevention, mitigation, suppression, management, and rehabilitation. The Department of Defense and other relevant federal departments and agencies must submit to the commission an inventory of surplus cargo and passenger aircraft and excess common-use aircraft parts that may be used for wildland firefighting purposes.