The Covid-19 pandemic has caused major changes to family life followed by a call for knowledge about how these changes have affected parental functioning, particularly in vulnerable families (Prime, Wade & Brooks, 2020). This study uses a natural experiment design to investigate the effect of the Covid-19 lockdown on parental mental health, parenting stress and three dimensions of destructive interparental conflict in a heterogeneous sample characterized by pre-existing relationship problems. Results from mixed model regression analyses showed that despite significantly higher levels of parenting stress in the lockdown group (n = 744 families) compared with the control group (n = 427 families), lockdown did not adversely affect parental mental health or levels of destructive conflicts behaviors. In fact, levels of verbal aggression and child involvement in conflict decreased during lockdown among parents living apart. Pre-existing destructive conflict levels, financial problems and age of youngest child did not moderate any of the associations. Thus, findings indicated that pre-existing family vulnerability did not predispose for reduced parental functioning during lockdown, beyond increased parenting stress. Resilient processes and an increased sense of purpose may be potential mechanisms. Caution should be taken when generalizing the findings due to the welfare context of the sample and as long-term lockdown effects were not investigated. Importantly, children in vulnerable families may have been negatively impacted by increased family time, despite relatively stable parental functioning during lockdown.