COVID-19 has resulted in a mental health crisis across the globe. Understanding factors that may have increased individuals’ risk of poor mental health outcomes is imperative. Individual differences in attachment styles have been shown to predict poorer mental health outcomes and insecure individuals struggle to cope with stressful situations. Therefore, we extended past research by examining whether higher attachment insecurity (anxiety and avoidance) predicted worse mental health over time during the COVID-19 pandemic. Study 1 examined mood and mood fluctuations daily over a week in the beginning of lockdown and depression and anxiety weekly over a five-week period (n = 200). Study 2 examined depression and anxiety before and during the pandemic (n = 100 couples). As predicted, individuals higher (vs. lower) in attachment anxiety, but not avoidance, reported greater depression and anxiety during the pandemic in both studies. Individuals lower in attachment anxiety experienced an improvement in mental health over time in Study 1 suggesting that more secure individuals may recover more quickly from the initial change in circumstances. Attachment styles did not significantly predict mood or mood fluctuations. Attachment anxiety is likely to be a risk factor for poor mental health outcomes during COVID-19.