National Security Diversity and Inclusion Workforce Act of 2021 This bill requires each national security agency to report on its diversity and inclusion efforts. The bill defines diversity as diversity of persons based on gender, race, ethnicity, disability status, veteran status, sexual orientation, gender identity, national origin, and other demographic categories. It also requires each such agency to develop a system to collect and analyze applicant employment data to identify areas for improvement in attracting diverse talent, with emphasis on senior and management positions; conduct periodic interviews with a representative cross-section of the national security workforce to obtain workplace information; sponsor workforce members to participate in a Senior Executive Service candidate development program or similar program; implement performance and advancement requirements for the workforce; create opportunities for senior personnel to participate in outreach events and to discuss issues relating to diversity and inclusion; and expand training on bias in the workplace and flexible work policies. The bill encourages agencies to expand professional development and career advancement opportunities that support their missions and to seek a diverse and talented pool of employment applicants by reaching out to educational organizations and professional associations.