Some scholars have argued urban autonomy creates inclusive spaces for migrants, while others have argued interiorized border security and discriminatory policing constrain racialized migrants. This article examines the extent to which the urban context represents a spatial mobility trap for urban undocumented youths as they become adults, based on data from longitudinal comparative ethnography with undocumented youths, schools, and civil society actors in Paris and New York City. Public schools, public transportation, policing, and informal labor market practices distinguish the urban regimes of migrant il/legality in the two cities, shaping how such youths move within and outside urban space. These elements may magnify or mitigate legal‐status distinctions between undocumented youths and others. As protections for undocumented minors waned at adulthood, New York City's regime blurred legal‐status distinctions, accelerating intracity mobility for most youths while Paris's approach brightened such distinctions, limiting mobility. These findings contribute a spatial mobility perspective to studies of migrant illegality and an urban sociology perspective to the transition to adulthood.