Background: Existing literature shows low and unequal access to mental health treatment globally, resulting in policy efforts to promote access for vulnerable groups. Yet, there is little evidence about how inequalities develop once individuals start treatment. The greater use of mental health care among individuals with low income, such as in the Dutch system, might be driven by differences in need and might not necessarily lead to better treatment outcomes. In this study, we aimed to examine income inequalities in four stages of the mental health treatment pathway while adjusting for need. Methods: We constructed a nationwide retrospective cohort study, examining all patients aged older than 18 years with a first specialist mental health treatment record in the Netherlands between 2011 and 2016, excluding those who did not receive any treatment minutes. We linked patient-level data from treatment records to administrative data on income, demographics from municipal registries, and health insurance claims. We used multivariate models to estimate adjusted associations between household income quintile (standardised for household size) and outcomes characterising four stages of mental health treatment: severity at baseline assessment based on the Global Assessment of Functioning (GAF) score, treatment minutes received, functional improvement by the end of the initial record, and additional treatment in a subsequent record. Estimates were adjusted for patient need (97 categories of primary diagnosis and severity at baseline assessment measured by GAF) and demographic covariates. Findings: Our study population consisted of 951 530 adults with a first specialist mental health treatment record in the Netherlands between Jan 1, 2011, and Dec 31, 2016. Patients in our cohort were on average aged 45·0 years (range 19–107) and mostly female (529 859 [55·7%] women and 421 671 [44·3%] men; no ethnicity data were available). First, we found that patients with the lowest income had the greatest initial therapist-assessed ...