Few studies have examined mental health variables in relation to social vs. non-social smartphone use, and how such types of use relate to problematic smartphone use (PSU). We conducted a web survey of 316 American undergraduate students about severity of depression, anxiety, fear of missing out (FOMO), social and non-social smartphone use, and PSU. Using structural equation modeling, we found that compared to social smartphone use, non-social use was more strongly linked with severity of PSU (β = 0.18, p < .05) and FOMO (β = 0.24, p < .05). FOMO mediated relations between depression severity and non-social smartphone use (β = 0.09, p = .04). Furthermore, FOMO mediated relations between both depression (β = 0.23, p < .001) and anxiety (β = 0.16, p = .04) with PSU severity. We discuss non-social smartphone use as a possible avoidance strategy among anxious individuals, and the role of FOMO in individuals desiring social connectedness but also feeling socially anxious. Results are discussed in the context of a comprehensive theoretical model on pathways toward excessive internet use.