Household income and caregiver mental health are important drivers of children’s health and development. The COVID-19 pandemic created huge economic and mental health disruptions. This study examines financial hardship and its relationship with caregiver and child mental health using Australia’s only representative data spanning three years of the pandemic. Analysis of the repeated, cross-sectional National Child Health Poll included 12,408 caregivers and 20,339 children over six waves (June 2020–April 2023). Caregivers reported their income (dichotomised into low versus not) and deprivation (missing one or more of eight essential items, versus not) and mental health for themselves (Kessler-6, poor versus not) and each child (Self-Rated Mental Health, poor/fair versus good/very good/excellent). Binary logistic models were fitted to predict marginal probabilities of mental health measures by low income and deprivation, over time. Results show that while low income decreased from 41% to 34% over the study period, deprivation increased from 30% to 35%. Poor mental health peaked with stay-at-home orders in 2021 before recovering. Caregivers experiencing low income or deprivation had higher rates of poor mental health throughout the study and slower recovery compared to those without financial hardship. Children in families experiencing financial hardship had slightly higher proportions of poor/fair mental health in 2021–2022, but they were mostly equivalent in June 2020 and April 2023 (range 6–8%). Addressing financial hardship may offer an avenue for improving caregiver mental health. This has implications for post-pandemic recovery and addressing contemporary issues of increasing cost of living and limited mental health supports and services.