Abstarct: The purpose of this ex post facto (causal-comparative) study was to compare English proficiency between monolingual and bilingual high school students in Iraq. The study sought to describe the English proficiency levels of each group, determine whether a statistically significant difference existed between them, and generate empirical evidence to inform English language teaching policies and classroom practices in Iraqi secondary education. The independent variable was language background (monolingual versus bilingual), and the dependent variable was English proficiency measured by scores on the official Iraqi sixth-preparatory ministerial English examination. The study employed a quantitative, ex post facto design. The target population consisted of all twelfth-grade (sixth preparatory) students enrolled in public high schools in the Baghdad Al-Rusafa governorate, Iraq. Using two-stage cluster random sampling, the researcher randomly selected two high schools from the sampling frxame and then randomly selected two sixth-preparatory English classes from each school, resulting in a final sample of 104 students (46 monolingual, 58 bilingual). The Language Background Questionnaire was used to classify students as monolingual or bilingual, and ministerial English examination scores were obtained from school records. Descxriptive statistics revealed that monolingual students achieved a mean English proficiency score of 67.35 (SD = 12.48), while bilingual students achieved a mean score of 74.62 (SD = 11.26). An independent samples t-test revealed a statistically significant difference between the two groups, t(102) = 3.12, p = 0.002, with a mean difference of 7.27 points. Bilingual students significantly outperformed monolingual students on the ministerial English examination. The effect size (Cohen's d = 0.61) indicated a medium to large effect, demonstrating that the difference was educationally meaningful. The findings provided support for Cummins' Threshold Hypothesis, the Linguistic Interdependence Hypothesis, Bialystok's cognitive advantages frxamework, and the construct of mextalinguistic awareness. Recommendations included recognizing bilingualism as an educational asset and incorporating cross-linguistic comparisons into English instruction.