Given the ubiquity and interactive power of smartphones, there are opportunities to develop smartphone applications (apps) that provide novel, highly accessible mental health supports. This paper details the development of a smartphone app, “MoodMission,” that aims to provide evidence-based Cognitive Behavior Therapy (CBT) strategies for mood- and anxiety-related problems, contributing to the prevention of clinically significant depression and anxiety disorders and serving as an adjunct to therapeutic interventions delivered by trained health professionals. MoodMission was designed to deliver strategies in the form of real-time, momentary responses to user-reported low moods and anxiety. The development process involved: (a) construction of a battery of strategies, (b) empirical evaluation, (c) a software and behavioral plan design and testing process, (d) user feedback, and (e) a public launch. A pilot study of 44 participants completed the Mobile Application Rating Scale (MARS; Hides et al., 2014) for usability testing and feedback. MoodMission was rated significantly higher than standardized health app norms on the majority of the domains, including Entertainment, Interest, Customization, Target Group, Graphics, Visual Appeal, Quality of Information, Quantity of Information, Visual Information, Credibility of Source, Recommendation to Use, Estimated Frequency of Use, and Overall Rating (Hedges's g range 0.57–1.97, p <.006). Case examples illustrate the practical uses of the app. In addition to clinical applications, MoodMission holds promise as a research tool either as an augmentation to clinician-delivered therapy, or as a vehicle for standardizing client access to specific CBT strategies (e.g., in studies intending to study different change processes).