OBJECTIVES: To determine mediators of improvements in infant safe-sleep (SS) practices in a mobile health intervention. METHODS: In a cluster-randomized controlled trial, mothers received SS intervention or breastfeeding control videos for 60 days. Maternal responses about infant sleep position and location (outcomes) and mediators (attitudes, perceived social norms, and perceived control) from the theory of planned behavior were assessed. Intervention effects on mediators and association between mediators and outcomes were examined. RESULTS: Of 1600 recruited, 1263 mothers participated. Mothers receiving SS videos were more likely to have positive attitudes and norms for supine sleep (attitudes: adjusted odds ratio [aOR] = 2.35 [95% confidence interval (CI) 1.72 to 3.20]; norms: aOR = 1.75 [95% CI 1.27 to 2.42]) and recommended sleep location (attitudes: aOR = 1.91 [95% CI 1.54 to 2.36]; norms: aOR = 1.37 [95% CI 1.13 to 1.66]). Positive attitudes and norms toward supine sleep and room-sharing without bed-sharing were associated with higher odds of both practices (supine: aOR = 8.25 [95% CI 4.72 to 14.43] for positive attitudes and aOR = 6.67 [95% CI 4.25 to 10.46] for norms; room-sharing: aOR = 7.14 [95% CI 5.35 to 9.53] for positive attitudes and aOR = 4.44 [95% CI 3.03 to 6.51] for norms). Both positive attitudes and positive norms mediated the effect of the intervention. CONCLUSIONS: The intervention achieved success in improving adherence to SS recommendations by changing maternal attitudes and norms about supine sleeping and room-sharing without bed-sharing. Recognition that these attitudes and norms appear to be the main drivers of mothers’ choices regarding infant-sleep practices should inform health messaging strategies to promote SS.