Background Equality of access to memory assessment services by older adults from ethnic minorities is both an ethical imperative and a public health priority. Objective To investigate whether timeliness of access to memory assessment service differs between older people of white British and South Asian ethnicity. Design Longitudinal cohort. Setting Nottingham Memory Study; outpatient secondary mental healthcare. Subjects Our cohort comprised 3654 white British and 32 South Asian older outpatients. Methods The criterion for timely access to memory assessment service was set at 90 days from referral. Relationships between ethnicity and likelihood of timely access to memory assessment service were analysed using binary logistic regression. Analyses were adjusted for socio‐demographic factors, deprivation and previous access to rapid response mental health services. Results Among white British outpatients, 2272 people (62.2%) achieved timely access to memory assessment service. Among South Asian outpatients, fourteen people (43.8%) achieved timely access to memory assessment service. After full adjustment, South Asian outpatients had a 0.47‐fold reduced likelihood of timely access, compared to white British outpatients (odds ratio 0.47, 95% confidence interval 0.23‐0.95, P value = .035). The difference became non‐significant when restricting analyses to outpatients reporting British nationality or English as first language. Older age, lower index of deprivation and previous access to rapid response mental health services were associated with reduced likelihood of timely access, while gender was not. Conclusions In a UK mental healthcare service, older South Asian outpatients are less likely to access dementia diagnostic services in a timely way, compared to white British outpatients.