Social and economic inequality are chronic stressors that continually erode the mental and physical health of marginalized groups, undermining overall societal resilience. In this comprehensive review, we synthesize evidence of greater increases in mental health symptoms during the COVID-19 pandemic among socially or economically marginalized groups in the United States, including (a) people who are low income or experiencing homelessness, (b) racial and ethnic minorities, (c) women and lesbian, gay, bisexual, transgender, queer, and questioning (LGBTQ+) communities, (d) immigrants and migrants, (e) children and people with a history of childhood adversity, and (f) the socially isolated and lonely. Based on this evidence, we propose that reducing social and economic inequality would promote population mental health and societal resilience to future crises. Specifically, we propose concrete, actionable recommendations for policy, intervention, and practice that would bolster five "pillars" of societal resilience: (1) economic safety and equity, (2) accessible healthcare, including mental health services, (3) combating racial injustice and promoting respect for diversity, equity, and inclusion, (4) child and family protection services, and (5) social cohesion. Although the recent pandemic exposed and accentuated steep inequalities within our society, efforts to rebuild offer the opportunity to re-envision societal resilience and policy to reduce multiple forms of inequality for our collective benefit.