Regular physical activity (PA) has shown to reduce the risk of several chronic diseases and improve physiological and psychological health, yet over half of the Canadian population remains inactive. Young adults, ages 25-44, show the sharpest declines in PA, but a paucity of research explaining the reasons for this deflection point exists. Recent research has indicated that life-transitions, such as marriage and parenthood, are probable reasons for the decline in PA, but little is known about how early career transitions impact PA status. Professional occupations have shown to be associated with the highest number of work hours per week and highest level of sedentary behaviour at work. The purposes of this study were to evaluate the changes in PA behaviour of new professionals across the early career transition using retrospective analysis and determine the critical correlates of changes in PA using the theory of planned behaviour. A Canada-wide sample of 267 new academic professors was examined. 30.7% of the sample reported meeting current PA guidelines. RM ANOVAs provided evidence that PA frequency (d = .36-.43) and total minutes (d = .39-.42) significantly declined across the transition to employment. PA levels across the transition were further attenuated after controlling for marital status, long work hours (>70 hrs/wk). The presence of young children in the home moderated the PA levels across the transition. The TPB explained 28-35% of PA behaviour (f2 = .39- .54), with intention and PBC emerging as independent predictors. Intention, in turn, was predicted by PBC, affective attitude, and instrumental attitude and explained 42% of the variance (f2 = .72). Perceptions of enjoyment and control based on limited time, inconsistent work schedule, heavy work demands, and pressure to reach tenure were critical correlates that distinguished between those who remained active across the transition from those who did not. The findings from this study highlight the importance for targeted PA interventions administered prior to the transition to professional employment to prevent habitual inactivity.