A growing proportion of young adults live with their parents in London, driven in part by high housing costs and weak wage growth. Meanwhile, reductions in driver’s license holding and car use among millennials have gained significant attention in transport research. A number of distinct lifestyle changes have been suggested to have contributed to these emerging trends. Millennials’ delayed transition to adulthood is often acknowledged, but few studies have explored how living with parent’s past adolescence affects travel behaviour. This study uses survey data from Transport for London (TfL) to compare the travel behaviours of millennials who live with parents with those who live independently. The results show that those living with parents travel less frequently, and by less active and sustainable modes, with implications for health, social exclusion, and sustainability. Path analysis, a form of structural equation modelling, is used to uncover the mediating impacts of car access, socio-demographic and spatial characteristics. Millennials in multi-generational households largely live in outer London and in areas of lower public transport accessibility. Contrary to findings of previous studies, millennials who live with their parents are more likely to have access to a car than their counterparts who live independently, although they are less likely to have learnt to drive. These results reveal wide variation in the travel behaviours of millennial Londoners, resulting from the interaction of cohort-specific and traditional determinants of travel demand. This challenges the implicitly assumed homogeneity of millennial travel behaviours in much of the literature. Further, this research demonstrates the need for transport planners to account for broader macro-economic uncertainty in their forecasts of travel demand. To ensure the continued growth of active and sustainable travel among young adults, policy interventions must span economic, housing and transport disciplines.