The identification of potential mechanisms of influence (mediators) of socio-economic status (SES) on walking for transport is important, because the likely opposing forces of influence may obscure pathways for intervention across different SES groups. This study examined individual, and perceived social and physical environmental mediators of the relations of individual- and area-level SES with walking for transport. Two mailed surveys, six months apart, collected data on transport-related walking and its hypothesized individual, social and environmental correlates. The sample consisted of 2194 English-speaking adults (aged 20-65) living in 154 Census Collection Districts (CCDs) of Adelaide, Australia. Individual-level SES was assessed using data on self-reported educational attainment, household income, and household size. Area-level SES was assessed using census data on median household income and household size for each selected CCD. Bootstrap generalized linear models examined associations between SES, potential mediators, and total weekly minutes and frequency of walking for transport. The product-of-coefficient test was used to assess mediating effects. Individual, social-environmental, and physical environmental factors significantly contributed to the explanation of the relations between SES and transport-related walking frequency. Educational attainment and area- and individual-level income played independent roles in explaining frequency of walking for transport, through opposing common and distinct pathways. While engagement in leisure-time physical activity was the most influential mediator of the association between educational attainment and frequency of walking for transport, the number of motorized vehicles and perceived levels of environmental aesthetics and greenery were the strongest mediators of the relations of frequency of transport-related walking with individual- and area-level income, respectively. Environmental interventions aimed at increasing residential density, reducing physical barriers to walking and traffic load, developing social-support networks, and creating greener and more aesthetically pleasing environments in more-disadvantaged areas may help to reduce SES inequalities in participation in physical activity, by facilitating walking for transport.