Objective: To estimate feasibility, usability and efficacy of a mobile parenting app (Rx for Success; RxS) to enhance reading guidance provided to parents of young children during well-visits. Methods: This trial was conducted at a clinic serving primarily families of Hispanic ethnicity and low-socioeconomic status (SES) where Reach Out and Read (ROR) is standard practice. It involved 252 parent-child dyads in two age groups (∼6-months-old, ∼18-months-old) randomized during well-visits to receive RxS or a children's book modeling alternatives to screen time (Control) by research coordinators. RxS involves videos, activities and “push” messages. Follow-up assessments were conducted approximately 6-months later, including impression and use, shared reading behaviors, child language and screen time. Results: A total of 217 dyads completed both visits (110 RxS, 107 Control). Time to introduce RxS was under 3 minutes and 32% of parents experienced largely minor performance issues. Parent impression of RxS was favorable for both age groups at baseline and follow-up, though use was infrequent, attributable to a desire for more relevant and updated content. Significant findings favoring RxS included shared reading as a favorite activity, more frequent shared reading reported at 12-months and higher language scores at 24-months. Screen time was equivalent between cohorts, exceeding AAP guidelines. Conclusions: A mobile app introduced to parents of young children from low-SES backgrounds was feasible during well-visits, rated as helpful, and effective to enhance shared reading at younger and language at older ages. While a potentially impactful enhancement to ROR, features needing improvement were identified.