Four distinct patterns of change in internalizing symptoms were identified, and the modal response was resilience.•71% of people experienced no change in internalizing symptoms during the pandemic. More people experienced improving (18%) than worsening (11%) internalizing symptoms during the pandemic. The majority of Irish adults have not experienced a negative mental health effect from the COVID-19 pandemic. Longitudinal data indicates that the mental health of the general population may not have been as badly affected by the COVID-19 pandemic as some had feared. Most studies examining change in mental health during the pandemic have assumed population homogeneity which may conceal evidence of worsening mental health for some. In this study, we applied a heterogeneous perspective to determine if there were distinct groups in the population characterised by different patterns of change in internalizing symptoms during the pandemic. Self-report data were collected from a nationally representative sample of Irish adults (N = 1041) at four time-points between April and December 2020. In the entire sample, mean levels of internalizing symptoms significantly declined from March to December 2020. However, we identified four distinct groups with different patterns of change. The most common response was ‘Resilience’ (66.7%), followed by ‘Improving’ (17.9%), ‘Worsening’ (11.3%), and ‘Sustained’ (4.1%). Belonging to the ‘Worsening’ class was associated with younger age, city dwelling, current and past treatment for a mental health problem, higher levels of empathy, and higher levels of loneliness. Sample attrition was relatively high and although this was managed using robust statistical methods, bias associated with non-responses cannot be entirely ruled out. The majority of adults experienced no change, or an improvement in internalizing symptoms during the pandemic, and a relatively small proportion of adults experienced a worsening of internalizing symptoms. Limited public mental health resources should be targeted toward helping these at-risk individuals.