In the past two decades European states have increasingly sought to stem irregular migration to Europe by cooperating with non-European countries to “externalize” migration and asylum management beyond European borders and territories. The externalization of migration policy amounts to delegating border control in third countries, enabling deportations of irregular migrants through readmission agreements (RA), while also including aid and development packages that address the drivers of migration in origin and transit countries. It is unclear, however, whether or under what conditions such policies have had their intended effects. In this paper, we aim to measure how migrant and refugee flows respond to such European externalization of migration and asylum management. Our analyses cover the period of the so-called “migrant” or “refugee crisis” of 2015, which offers a unique context in which to study the relationship between migration policies and flows. After introducing a new database on external migration policy instruments, we use Frontex data to describe the patterns in irregular migration flows, focusing on the spatial and categorical distribution of migrants and refugees from 2009 to 2018. We first introduce a useful distinction between “likely irregular migrants” and “likely refugees”, who both cross borders illegally into Europe. We then explore the extent to which the closure of migration routes led to the rerouting of migration flows. We show that such a phenomenon is actually limited to few nationalities. We eventually argue that externalization policies, although partially effective at reducing the overall number of irregular border crossings into Europe, directly affect “likely refugees” who remain stranded in transit countries or who renounce fleeing their country of origin. Then, we present key policy trends across European states and third countries in regard with externalization. We first argue that the 2015 crisis reinforced existing dynamics of diplomatic engagement and cooperation with third countries in matters of migration and asylum and extended their geographical scope. We then examine the impact of policies on irregular flows. Focusing on the impact of readmission agreements, we contend that bilateral agreements are not effective in limiting irregular migration from the signing countries of origin. However, we also observe that bilateral implemented protocols of EU readmission agreements (EU RA) are associated with fewer irregular border crossings from the relevant third countries. We suggest that this effect can be explained by pre-existing political relations between the parties to the EU readmission agreements and the limited number of irregular border crossers from these nationalities. Overall, our findings call into question the appropriateness of externalization policies as tools for addressing migration issues, in particular large refugee flows in times of crisis.