As mobile technology allows users to be online anywhere and at all times, a growing number of users report feeling constantly alert and preoccupied with online streams of online information and communication—a phenomenon that has recently been termed online vigilance. Despite its growing prevalence, consequences of this constant orientation toward online streams of information and communication for users' well-being are largely unclear. In the present study, we investigated whether being constantly vigilant is related to cognitive consequences in the form of increased mind-wandering and decreased mindfulness and examined the resulting implications for well-being. To test our assumptions, we estimated a path model based on survey data (N = 371). The model supported the majority of our preregistered hypotheses: online vigilance was indeed related to mind-wandering and mindfulness, but only mindfulness mediated the relationship with decreased well-being. Thus, those mentally preoccupied with online communication were overall less satisfied with their lives and reported less affective well-being when they also experienced reduced mindfulness.