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 representing 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. Methods. 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.