OBJECTIVE: To describe the development and testing of an electronic HEalthy EnviROnments study mobile application (app) with parents of preschoolers using a user-centered design approach. DESIGN: An iterative, multiphase approach comprising formative research, intervention mapping, app development and testing, and pilot testing. SETTING: Online. PARTICIPANTS: Families with limited resources who have a preschooler enrolled in Head Start in the 2 states. INTERVENTION: Twelve-week digital intervention embedded within the social ecological model and employing the ecocultural family theory and Social Cognitive Theory. MAIN OUTCOME MEASURES: Formative research provided insights into parents' use of digital devices and potential intervention features. The findings will inform intervention mapping and wireframe (app schematic) testing to examine the app's functionality, usability, and user experience. A pilot study (n = 30) will be conducted to assess the feasibility, acceptability, and usability of the electronic HEalthy EnviROnments study mobile app, along with theoretical mediators, as well as behavioral and implementation outcomes. ANALYSIS: The results will be interpreted iteratively to inform subsequent study phases. Quantitative and qualitative analyses will be used to assess the feasibility of a digital parent intervention to enhance parents' feeding and activity parenting practices and mindful parenting, with the long-term goal of improving preschoolers' eating and activity behaviors in their home environment.