This study examines the developmental impact of excessive smartphone use on cognitive functioning and externalizing behaviors in elementary school children. Specifically, it examines the relationship between smartphone overuse and cognitive skills related to future thinking, as well as related mental health outcomes. A descriptive cross-sectional design was utilized. Data were collected from 200 Iranian parent-child pairs (139 boys and 61 girls) aged between 6–13 years. Parents completed the Children’s Future Thinking Questionnaire (CFTQ) to assess related cognitive functions to future thinking and the Child Behavior Checklist (CBCL) to evaluate mental health outcomes. Additionally, Smartphone overuse was measured through Child Smartphone Addiction Questioner (CSAQ).  The results revealed significant negative correlations between smartphone addiction and key cognitive functions such as episodic foresight, prospective memory, and planning. Notably, smartphone addiction was strongly correlated with increased externalizing behaviors, including aggression and rule-breaking, and other mental health problems, such as anxiety and depression. Regression analysis showed that smartphone addiction accounted for 25% of the variance in these negative outcomes, highlighting its significant impact on child development. These findings highlight the need for interventions that address smartphone addiction and focus on strengthening future-oriented cognitive skills, which are essential for the improvement or prevention of mental health and behavioral outcomes in children.