The capacity of the workforce lags behind the current demand for timely autism diagnostic assessment. Primary care providers (PCPs) are well-positioned to diagnose autism at earlier ages than providers from other disciplines. Thus, bolstering PCPs' diagnostic capabilities has been the focus of many recent capacity-building initiatives. Using data from the National Survey of Children's Health, this study aimed to evaluate whether diagnosis of autism in primary care has changed over time and whether diagnosis in primary care relates to age at autism diagnosis. Results indicated that the likelihood of being diagnosed with autism by a PCP decreased by about 2% with every passing year from 2004 to 2019 when controlling for demographic characteristics. PCPs diagnosed children approximately 1 year earlier than non-PCPs (e.g., psychiatrists and psychologists), which supports the critical role PCPs can play in timely diagnosis. Further research is needed to understand why the proportion of children diagnosed by PCPs has decreased over time; however, these findings suggest that capacity-building initiatives have not yet reached community practice. Future research should focus on the dissemination and implementation of training initiatives in community-based primary care practices.