Aims: Mobile gaming is a dominant form of gaming, known for its portability and for game characteristics that motivate continuous play and spending. Such involvement may also turn problematic, but research on problem gaming (PG) has tended to focus on non-mobile forms of gaming. The study was based on a cross-sectional observational design where students in upper secondary schools were recruited to a survey about mobile gaming. The age of the respondents ranged from 16 to 23 years ( n = 519; 52.4% men; mean age = 17.2 years, SD = 1.1). Methods: We examined (1) gaming frequency, gaming contexts, and in-game spending in relation to PG; (2) gaming context in relation to academic achievement and sleep quality; and (3) PG according to gaming platform (i.e., playing on mobile, console/computer, or mixed platforms) with Kruskal−Wallis tests, chi-square tests and Spearman rank-order correlations. Results: PG was positively associated with mobile gaming hours per week (η2 = .02, p < .01), minutes per session (η2 = .03, p < .001), making in-app purchases (Cramer's V = .15, p < .05), and gaming during homework (Cramer's V = .14, p < .05). Statistically significant associations were found between mobile gaming in bed and later sleep midpoint for weekdays ( rs = .18, p < .001) and weekends ( rs = .11, p < .05). Mixed platform gamers had increased likelihood of PG, console/computer gamers had increased likelihood of being at risk for PG, and mobile gamers had lower risk for PG (Cramer's V = .18, p < .001). Conclusion: Future studies should include specific measures of mobile gaming as it appears implicated in problem gaming, albeit to a lesser degree than console and computer gaming.