Research demonstrates that depression and anxiety symptom severity are related to problematic smartphone use (PSU). However, less is known about variables mediating these relationships. This study aimed to test whether proneness to boredom increased PSU. We also tested whether boredom proneness mediates relations between both depression and anxiety symptom severity with PSU. Using a cross-sectional design, we surveyed 298 American college students about their frequency of smartphone use, levels of PSU, depression, anxiety, and boredom proneness. Using structural equation modeling, we modeled depression and anxiety symptom severity predicting boredom proneness, in turn predicting levels of PSU and smartphone use frequency (SUF). Results demonstrate that boredom proneness predicted PSU, but not SUF. Boredom proneness mediated relations between both depression and anxiety symptom severity with PSU levels (but not usage frequency). We discuss the phenomenon in terms of depressed or anxious college students having difficulty attending to their schoolwork, subsequently experiencing boredom, and engaging in PSU to relieve their boredom.