Examining processes that characterize the ebb and flow of emotions offers insight into how infants modulate their own emotional experience as well as how both mothers and infants jointly regulate their emotional states. Drawing from polyvagal theory, which posits that vagal tone supports the capacity to quickly, flexibly, and adaptively respond to contextual demands (Porges, 2003, 2007), I hypothesized that infants with greater vagal tone (indexed by respiratory sinus arrhythmia; RSA) would show stronger evidence of emotion regulation and coregulation processes during free play and a frustrating task at 24 weeks child age. To evaluate these hypotheses, I used dynamic structural equation modeling (DSEM; Asparouhov, Hamaker, & Muthén, 2018) to examine biologically-based differences in second-by-second infant emotion regulation (equilibria, volatility, carryover, and feedback loops in positive and negative affect engagement) and mother- and infant-driven coregulation processes, among a sample of 210 low-income, Mexican-origin mother-infant dyads. Results offered evidence of both mother-driven and infant-driven emotion coregulatory processes during free play, which did not differ based on infant RSA. Results offered limited support for RSA-based differences in infant self-regulation processes during the teaching task, such that infants with below average RSA tended to respond to increased negative affect with subsequent increases in positive affect engagement. Prenatal maternal depressive symptoms also accounted for greater infant emotional volatility and weaker mother-driven emotion coregulation. Results highlight the unique roles mothers and infants play in achieving emotion regulation, as well as between-dyad differences in these processes, suggesting multiple pathways towards resilience among low-income, Mexican-origin families.