Objective: Past behavior has been consistently shown to predict and explain future behavior. It has been proposed that past behavior effects reflect both reasoned action and automatic processes. The current study sought to explore the mediation of past behavior-future behavior relationship via constructs reflecting these processes across three populations and behaviors: binge drinking in university students, flossing in adults, and parental sun safety behavior of children 2 – 5 years of age. Furthermore, this study used a measure of past behavior that combined long-term, recent, and routine patterns of behavioral engagement. Method: A prospective design with two waves of data collection spaced six weeks apart was adopted. Participants (total N = 754) completed an initial survey containing measures of past behavior (frequency, recency, and routine), social cognition (attitudes, subjective norms, perceived behavioral control), and behavioral automaticity. Six weeks later, participants (N = 454) completed a self-report measure of behavior and behavioral automaticity. Results: Structural equation modelling revealed that automatic, but not reasoned-action processes, mediated the past-to-future relationship, across the three behaviors. Results further revealed that long-term, recent, and routine patterns of behavioral engagement were highly correlated and indicated a second-order past behavior latent variable. Conclusions: While both reasoned-action and automatic factors can predict a range of health behaviors, automatic processes appear to explain the effect of past behavior on future behavior. Further investigations should focus on exploring the role of other non-conscious and automatic processes such as counter-intentional habits and implicit beliefs in explaining engagement in heath behaviors.