With a rapidly growing older population, understanding factors that contribute to healthy ageing is a public health priority - and one such factor is mobility. Using cognitive assessments and magnetic resonance imaging (MRI), this thesis aimed to characterise the associations between mobility and the ageing brain. First, a systematic review and meta-analysis of the literature was conducted to examine the relationship between different domains of cognition (memory, executive function and processing speed) and mobility (gait, balance and lower-extremity function). The reviewed evidence suggested that reduced mobility was correlated with lower cognitive function in older adults, although notable variability was observed between studies. To address the gaps identified by the review, I examined mobility-cognition associations in the Canadian Longitudinal Study on Ageing. Findings supported the hypothesis that poor mobility is associated with poor cognition, and indicated that this relationship is not domain-specific. In addition, I found that controlling for a variety of age-related confounders did not nullify the mobility-cognition association. Less is known about the neural correlates of poor mobility. To examine which brain structures are associated with mobility, voxel-based-morphometry and diffusion tensor imaging outcomes were correlated with mobility measures in a sample of older adults from the Whitehall II cohort. Our findings indicated that older adults with poor mobility tend to have lower grey matter volume and reduced white matter microstructure. Since mobility relates to brain structure and function, it is plausible that improving mobility may lead to cognitive and structural benefits. To test this, I examined the effects of a physical activity intervention in a sub-sample recruited from the Retirement in Action study. In a between-group comparison of change over 6-months, we did not find evidence that physical activity modified cognitive or MRI outcomes. The evidence presented in this thesis supports the hypothesis that mobility is positively associated with cognition and brain structure in healthy older adults. Contrary to predictions, however, we did not observe exercise-induced effects in brain structure or function.