Internationally, stresses related to the COVID-19 pandemic negatively affected the mental health of family caregivers of adults with intellectual and developmental disabilities (IDDs). This cross-sectional study investigated demographic, situational and psychological variables associated with mental wellbeing among family caregivers of adults with IDDs during the COVID-19 pandemic. Baseline data from 202 family caregivers participating in virtual courses to support caregiver mental well-being were collected from October 2020 to June 2022 via online survey. Mental well-being was assessed using total scores from the Warwick-Edinburgh Mental Wellbeing Scale. Demographic, situational and psychological contributors to mental well-being were identified using hierarchical regression analysis. Variables associated with lower levels of mental well-being were gender (women); age (<60 years old); lack of vaccine availability; loss of programming for their family member; social isolation; and low confidence in their ability to prepare for healthcare, support their family member's mental health, manage burnout and navigate healthcare and social systems. Connection with other families, confidence in managing burnout and building resilience and confidence in working effectively across health and social systems were significant predictors of mental well-being in the final regression model, which predicted 55.6% of variance in mental well-being (P < 0.001). Family caregivers need ways to foster social connections with other families, and support to properly utilise healthcare and social services during public health emergencies. Helping them attend to their needs as caregivers can promote their mental health and ultimately improve outcomes for their family members with disabilities.