Do day-to-day emotions, social interactions, and sleep play a role in determining which anxious youth respond to supportive child-centered therapy (CCT) versus cognitive behavioral therapy (CBT)? We explored whether measures of day-to-day functioning (captured through ecological momentary assessment, sleep diary, and actigraphy), along with clinical and demographic measures, were predictors or moderators of treatment outcome in 114 anxious youth randomized to CCT or CBT. We statistically combined individual moderators into a single, optimal composite moderator to characterize subgroups for which CCT or CBT may be preferable. The strongest predictors of better outcome included: (a) experiencing higher positive affect when with one’s mother and (b) fewer self-reported problems with sleep duration. The composite moderator indicated that youth for whom CBT was indicated had: (a) more day-to-day sleep problems related to sleep quality, efficiency, and waking, (b) day-to-day negative events related to interpersonal concerns, (c) more DSM-IV anxiety diagnoses, and (d) college-educated parents. These findings illustrate the value of both day-to-day functioning characteristics and more traditional sociodemographic and clinical characteristics in identifying optimal anxiety treatment assignment. Future studies will need to enhance the practicality of real-time measures for use in clinical decision making and evaluate additional anxiety treatments.