Young people with childhood adversity (CA) were at increased risk to experience mental health problems during the COVID-19 pandemic. Pre-pandemic research identified high-quality friendship support as a protective factor that can buffer against the emergence of mental health problems in young people with CA. This longitudinal study investigated friendship buffering effects on mental health symptoms before and at three timepoints during the pandemic in 102 young people (aged 16–26) with low to moderate CA. Multilevel analyses revealed a continuous increase in depression symptoms following the outbreak. Friendship quality was perceived as elevated during lockdowns and returned to pre-pandemic baseline levels during reopening. A stress-sensitizing effect of CA on social functioning was evident, as social thinning occurred following the outbreak. Bivariate latent change score modeling revealed that before and during the pandemic, young people with greater friendship quality self-reported lower depression symptoms and vice versa. Furthermore, sequential mediation analysis showed that high-quality friendships before the pandemic buffered depression symptoms during the pandemic through reducing perceived stress. These findings highlight the importance of fostering stable and supportive friendships in young people with CA and suggest that through reducing stress perceptions high-quality friendships can mitigate mental health problems during times of multidimensional stress.