Marijuana-tobacco co-use has increased recently, particularly in young adults. We conducted a mixed-methods study to: (1) examine reasons for co-use; and (2) develop a scale assessing reasons for co-use among participants in a longitudinal cohort study of 3,418 students aged 18-25 from 7 Georgia colleges and universities. Phone-based semi-structured interviews were conducted in Summer 2015 among 46 current (past 30-day, n = 26) or lifetime (n = 20) marijuana users. Subsequently, scale items were developed and included at Wave 3. Participants reporting past 4-month tobacco and marijuana use (n = 328) completed the Reasons for Marijuana-Tobacco Co-use section. Per qualitative data, reasons for marijuana-tobacco co-use included synergistic effects, one triggering or preceding the other's use, using one to reduce the other's use, co-administration, social context, and experimentation. The survey subsample included 37.1% who used cigarettes, 30.4% LCCs, 9.4% smokeless, 23.7% e-cigarettes, and 30.4% hookah. Four subscale factors emerged: (1) Instrumentality, indicating synergistic effects; (2) Displacement, indicating using one product to reduce/quit the other; (3) Social context, indicating use in different settings/social situations; and (4) Experimentation, indicating experimentation with both but no specific reasons for co-use. These subscales demonstrated distinct associations with tobacco type used; nicotine dependence; marijuana and alcohol use frequency; tobacco and marijuana use motives, respectively; perceptions of tobacco and marijuana; and parental and friend use. Including these subscales in regressions predicting nicotine dependence and days of marijuana use significantly contributed to each model. These findings might inform theoretical frameworks upon which marijuana-tobacco co-use occurs and direct future intervention studies.