This study investigated caregiving responsibilities and associated mental health outcomes in young adult carers during the COVID-19 pandemic and had three aims: (1) to investigate differences in caregiving responsibilities across two groups of young adult carers (parental illness context vs. ill non-parent family member context) relative to non-carers, (2) to identify COVID-19/lockdown correlates of caregiving responsibilities, and (3) to examine the longitudinal associations between caregiving responsibilities and mental health outcomes. Of the 1048 Italians aged 18–29 (Mage = 24.48, SDage = 2.80; 74.33% female) who consented to complete online surveys at Time 1, 813 reported no ill family member (non-carers). Young adult carers included 162 with an ill parent and 73 with an ill non-parent family member. The study included 3 time points: 740 participants completed Time 2 assessment (Mage = 24.35, SDage = 2.81; 76.76% female), while 279 completed Time 3 assessment (Mage = 24.78, SDage = 2.72; 79.93% female). Key variables measured were 13 COVID-19/lockdown factors at Times 1 and 2, caregiving responsibilities at Time 2, and mental health outcomes at Time 3 (fear of COVID-19, anxiety, depression, wellbeing). Two COVID-19/lockdown factors were significantly correlated with higher caregiving responsibilities: insufficient home space, and greater time spent working and learning from home. As predicted, young adult carers reported higher caregiving responsibilities than non-carers, and this effect was greater in young adults caring for an ill parent compared to young adults caring for an ill non-parent family member. As expected, irrespective of family health status, caregiving responsibilities were longitudinally related to poorer mental health outcomes, operationalised as higher fear of COVID-19, anxiety, and depression, and lower wellbeing. Elevated young adult caregiving is an emerging significant public health issue that should be addressed through a multipronged approach that includes education about young adult carer needs for personnel across all relevant sectors and flexible care plans for ill family members that include a ’whole family’ biopsychosocial approach.