Active travel is an accessible form of physical activity, associated with physical and mental health benefits, particularly for older adults. This study aimed to identify the behavioural determinants that significantly impact older adults’ engagement in active travel as well as how and why these behavioural determinants occur. Employing an explanatory sequential mixed methods design informed by the Theoretical Domains Framework (TDF), the study focused on older adults in Scarborough, Canada. In the quantitative phase, a TDF-informed survey was distributed among Scarborough older adults. In the qualitative phase, select survey participants were interviewed about their experiences with these TDF determinants. To integrate the quantitative and quantitative results, data were interpreted and integrated using a joint display matrix. The survey results suggest that the TDF domains of motivation (i.e., feeling inspired to active travel) and skill (i.e., having the aptitude to active travel) were highly predictive of active travel behaviour. The likelihood of active travel increased if participants reported being encouraged and satisfied by their health, fitness, exercise, and well-being in general. The interview results suggest that fostering motivation and skill require a network of interventions that address multiple TDF domains such as promoting action planning, creating commitment, building social connectedness, increasing awareness of active travel options, and building emotional and physical safety. By integrating the quantitative and qualitative results, three overarching themes were presented about older adults’ needs to engage in active travel: building reassurance (i.e., putting safety first); building meaningfulness (i.e., creating joy, fulfillment, and purpose); and building relatedness (i.e., creating awareness and connection). The mixed methods study design employed in this study in conjunction with a novel application of the TDF allowed for a more thorough understanding of behavioural determinants of active travel among older adults.