This study investigated the impact of English language proficiency on students' academic performance in secondary schools in Uganda, with specific reference to Kasubi Parents Secondary School in Kasubi Parish, Rubaga Division, Kampala District. English serves as both the medium of instruction and the language of examination in Uganda's secondary education system, making English proficiency a critical determinant of academic outcomes across all subjects. The study was guided by objectives to assess students' English language proficiency levels, examine the relationship between English proficiency and academic performance in various subjects, and identify factors that influence English language acquisition among secondary school students. Using a cross-sectional survey design, data were collected from 180 respondents comprising students, English language teachers, and school administrators. Structured questionnaires, classroom observations, and standardized English proficiency assessments were used as instruments. Findings reveal a strong positive correlation (r=0.74, p<0.001) between English language proficiency and academic performance across subjects. Key determinants of English proficiency include home language environment, teacher quality, reading habits, and access to English language media. The study recommends intensification of English language instruction, establishment of reading programs, and strengthening of teacher professional development.