Diversity in Defense Act of 2020 This bill adds certain diversity-related requirements for the U.S. Armed Forces, including (1) the appointment of a Chief Diversity Officer in military components of the Department of Defense (DOD) and in the Coast Guard; (2) the establishment of a Diversity and Inclusion Advisory Council in DOD to, among other things, align diversity and inclusion with DOD strategic goals; (3) enhanced reporting on diversity and inclusion; (4) requiring a mentoring and career counseling program; (5) developing plans to increase recruitment and retention of female and minority members in the Armed Forces, (6) establishing in DOD the Office of the Special Inspector General for Racial and Ethnic Disparities in the Armed Forces to examine the effect of military justice and personnel policies on racial and ethnic disparities; (7) tracking investigations of supremacist, violent extremist, and criminal gang activity in the Armed Forces; and (8) improving responses to pregnancy and childbirth for members of the Armed Forces.