This paper investigates everyday geography of young adults and the unequal importance which spatial accessibility to a range of urban services may have for their mental health in order to identify those who are 'truly disadvantaged'. While the literature on the socially differentiated vulnerability to place effects has traditionally focused on the neighbourhood of residence, we consider daily activity locations to explore if socially disadvantaged populations are more exposed to ('differential exposure') and/or more affected by ('differential effect') low spatial accessibility to services compared to their more advantaged counterparts. Data came from 1,983 young adults (18-25 years-old) living in Montreal, Canada. We observed that less educated young adults had a lower spatial accessibility to services in their activity space than their more educated counterparts, but also that they were more vulnerable to having lower numbers of services in their surroundings: lower service accessibility in the activity space was associated with poorer mental health among less educated young adults but not among the more educated. We suggest three socio-spatial mechanisms related to (i) place experiences, (ii) flexibility in spatial behaviour and (iii) rules regulating actual access to services to explore why the 'objective' lack of services close to residential and activity locations may represent a greater burden to more socially disadvantaged people.