College students are among the most vulnerable groups to problems associated with high-risk drinking consequences such as illness, injury, sexual abuse, and death. Promising mobile health (mHealth) approaches, such as smartphone (SP) apps, can be used in interventions to address or prevent excessive drinking. : The aim of the investigation was to examine the efficacy of a theoretically based mHealth SP app for alcohol intervention in two independent samples (=379): Mandated participants (Study 1) and voluntary participants (Study 2). Study 1 included a controlled trial with Mandated participants randomized into either an in-person Brief Motivational Interviewing BMI (=70) or BMI + SP app intervention (=71). Study 2 included Voluntary participants who participated in either a Control group (=157) or the BMI + SP app intervention (=81). Participants in both studies completed baseline and 6-week assessments. : In Study 1, peak Blood Alcohol Concentration (BAC) of participants in the in-person BMI group had increased slightly at six weeks, while it had decreased for the app-based BMI + SP group. Study 2 participants using the BMI + SP app reported significant reductions in drinking and consequences; there were no changes in the (AO) Control group. The BMI + SP app was effective with both Mandated and Voluntary participants. Future testing with the BMI + SP app is needed to assess whether reach, adoptability, portability, and sustainability are greater with the mHealth smartphone app for alcohol intervention than in-person approaches.