Mobile devices are now widely used, including by young children in even the poorest communities, but too little research has examined their effects. Past research suggests that mobile media may affect school readiness, and that this relation could depend on the content used. We examined the relation between mobile media use and a composite school readiness measure of preliteracy, emergent math, and executive functioning, in low-SES preschoolers. Parents reported that children most frequently used mobile media to watch video content, and that few of the apps used by their children were educational. As predicted, time spent using mobile media predicted poorer school readiness skills, especially executive functioning. Parent reported use of educational apps was associated with stronger executive functioning, but otherwise we did not find predicted moderation effects of educational app use. Overall, study results support efforts to limit and improve the quality of screen time of preschoolers.