Prosocial and health protective behaviors are critical to contain the COVID-19 pandemic, yet adolescents have been difficult to engage. Attachment security promotes adolescents' capacities to navigate stress, and influences prosocial and health behaviors. Drawing on a diverse sample of 202 adolescents (48% female; 47.5% Latinx) this study evaluated relations among attachment, mental health, and prosocial and health protective responses to the COVID-19 pandemic. Attachment security (age 12) predicted adolescents' (age 15) COVID-19 prosocial (f[superscript 2] = 0.201) and health protective behaviors (f[superscript 2] = 0.274) during the pandemic via smaller-than-expected increases in mental health symptoms above pre-pandemic levels (age 14). Findings highlight the importance of attachment for supporting adolescents' mental health responses to life stressors and promoting prosocial and health protective behaviors.