Once a luxury, mobile devices are now utilized by most members of society, including those in even the poorest communities. Unfortunately, little research has examined the effects of mobile media use in young children, and even less on young children from low-SES communities. Past research on television, and preliminary research on mobile technology, suggests that mobile media may affect school readiness, and that the direction and strength of this relation could depend on the content and context of the use. The current study examined the relation between mobile media use and a composite school readiness measure that included preliteracy, emergent math, and executive functioning, in a sample of low SES preschoolers. We found that weekly mobile media time significantly predicted poorer school readiness skills, which was predicted given the scarcity of high-quality apps for preschoolers. This relation was especially clear in regard to preschoolers’ executive functioning, which had not been previously examined. While the effects of content and context of the usage were examined, few relationships emerged, perhaps due to measurement issues. The main results are concerning because children in this sample already have poorer school readiness than the general U.S. population, and their parents report considerable screen use. The results support efforts to limit screen time of preschoolers and are a step towards understanding the complicated relation between achievement and mobile technology use in preschoolers.