Multiple theoretical frameworks posit that interactions between the autonomic nervous system and higher-order neural networks are crucial for cognitive regulation. However, few studies have simultaneously examined autonomic physiology and brain activity during cognitive tasks. Such research is promising for understanding how early adversity impacts neurocognitive development in children, given that stress experienced early in life impacts both autonomic function and regulatory behaviors. We recorded event-related potentials (ERPs) as a neural measure of auditory selective attention, and cardiovascular measures of high-frequency heart rate variability (HF-HRV) and preejection period (PEP), in 105 3–5-year-old children with varying degrees of socioeconomic risk. First, we replicated a previous study from our lab: Increased socioeconomic risk was associated with larger ERP amplitudes elicited by distracting sounds. Next, we tested whether PEP and HF-HRV (at rest and during the task) were associated with the distractor ERP response, and found that a physiological profile marked by heightened sympathetic nervous system activity, indexed by shorter PEP, was associated with better ERP suppression of distractor sounds in lower SES children. Finally, we found that PEP mediated the relationship between socioeconomic risk and larger ERP responses to distractor sounds. In line with similar reports, these results suggest that for lower SES children, there is a potential biological cost of achieving better cognitive performance, seen here as increased cardiovascular arousal both at rest and in response to task demands.