In this retrospective cohort study using data from an integrated primary care and subspecialty network, we examined medical records of children seen in primary care at eligible autism spectrum disorder (ASD) screening ages and followed through at least 4 years of age. We examined the prevalence of ASD; age of first documented ASD diagnosis; and whether the prevalence and age of documented diagnosis varied by race, ethnicity, socio-economic status (SES) and site of care (urban versus suburban/rural). The prevalence of ASD across the cohort was 3.2%, with a median age of diagnosis of 3.93 years. ASD prevalence was unexpectedly higher among Asian children, non-Hispanic Black children, children with higher Social Vulnerability Index scores (a neighborhood-level proxy of socio-economic risk), and children who received care in urban primary care sites. There were no statistically significant differences in age at which ASD diagnosis was documented across socio-demographic groups. Receiving primary care at an urban site accounted for most other socio-demographic differences in ASD prevalence rates, except among Asian children, who were found to have higher adjusted odds of ASD diagnosis compared to White children (aOR = 1.82, p < 0.001). Determining what clinical-, individual- or systems-level factors contribute to ASD diagnosis remains important to improve equity.