In the US, unaccompanied migrant children and adolescents (hereinafter referred to as children) are predominantly from Central America's Northern Triangle. While unaccompanied migrant children are at high risk for psychiatric sequelae due to complex traumatic exposures, longitudinal investigations of psychiatric distress after resettlement are lacking. To identify factors associated with emotional distress and longitudinal changes in emotional distress among unaccompanied migrant children in the US. For this retrospective cohort study, the 15-item Refugee Health Screener (RHS-15) was administered between January 1, 2015, and December 31, 2019, to unaccompanied migrant children as part of their medical care to detect emotional distress. Follow-up RHS-15 results were included if they were completed before February 29, 2020. Median follow-up interval was 203 days (IQR, 113-375 days). The study was conducted in a federally qualified health center that provides medical, mental health, and legal services. Unaccompanied migrant children who completed the initial RHS-15 were eligible for analysis. Data were analyzed from April 18, 2022, to April 23, 2023. Traumatic events before migration, during migration, during detention, and after resettlement in the US. Emotional distress, including symptoms of posttraumatic stress disorder, anxiety, and depressive symptoms, as indicated by the RHS-15 (ie, score ≥12 on items 1-14 or ≥5 on item 15). In total, 176 unaccompanied migrant children completed an initial RHS-15. They were primarily from Central America's Northern Triangle (153 [86.9%]), were mostly male (126 [71.6%]), and had a mean (SD) age of 16.9 (2.1) years. Of the 176 unaccompanied migrant children, 101 (57.4%) had screen results above the positive cutoff. Girls were more likely to have positive screen results than boys (odds ratio, 2.48 [95% CI, 1.15-5.34]; P = .02). Follow-up scores were available for 68 unaccompanied migrant children (38.6%). On the follow-up RHS-15, most scored above the positive cutoff (44 [64.7%]). Three-quarters of unaccompanied migrant children who scored above the positive cutoff initially continued to have positive scores at follow-up (30 of 40), and half of those with negative screen scores initially had positive scores at follow-up (14 of 28). Female vs male unaccompanied migrant children (unstandardized β = 5.14 [95% CI, 0.23-10.06]; P = .04) and initial total score (unstandardized β = 0.41 [95% CI, 0.18-0.64]; P = .001) were independently associated with increased follow-up RHS-15 total score. The findings suggest that unaccompanied migrant children are at high risk for emotional distress, including symptoms of depression, anxiety, and posttraumatic stress. The persistence of emotional distress suggests that unaccompanied migrant children would benefit from ongoing psychosocial and material support after resettlement.