Background Informal care can affect the mental health of caregivers. The COVID-19 pandemic precipitated many people into informal care. Little is known about the longitudinal effect of informal care throughout the pandemic. We investigate changes in mental health in relation to changes in informal care between July 2020 and September 2021. Methods Using data from Understanding Society, we applied fixed-effects modelling to assess mental health variations associated with changes in caregiving among 13 557 participants (50 430 observations). Hours of weekly care were categorized as 0, 1–19, ≥20. Mental health was measured using the General Health Questionnaire (GHQ-12) as a continuous score and a binary indicator. Main analyses were stratified by gender. Results Compared to when delivering 0 hours care/week, the GHQ-12 scores of women providing care for 1–19 hours/week were 0.46 points higher (95%CI: −0.11, 1.09), while their mental health scores were 0.99 higher (95%: 0.08, 1.90) when caring for ≥20 hours/week. Changes on the binary GHQ-12 measure were only evident for women when providing ≥20 hours of weekly care. These changes were not substantial among men. Conclusion Informal care adversely impacted the mental health of women carers during the COVID-19 pandemic. Support programmes for informal carers should focus on alleviating caregiving loads in women.