The study assessed Lagos State public senior secondary school students’ English composition writing,
established their academic performances, and the relationships among the students’ academic performances in content, organisation, expression, mechanical accuracy as well as in letter and essay writing. Two research questions and two hypotheses guided the study. Descriptive survey and correlational research designs were adopted. The population comprised all public senior secondary two students in Lagos State. The multi-stage sampling technique was used to select 540 students from three educational districts in the state. Two composition writing tests (letter and essay writing) were used for the data collection. Data were analysed using Pearson Product Moment Correlation. Results that at least 97.3% of the students failed the test in letter and essay writing, at least 92% of the students failed in terms of content, organization and expression, and not even one student passed the test in terms of mechanical accuracy. Also, there were strong positive significant relationships among the students’ academic performances in letter and essay writing, as well as in all the assessment variables, except in mechanical accuracy where the positive significant relationships were very weak. Hence, it was recommended among others that teachers and parents/guardians of public senior secondary school students in Lagos State, Nigeria should pay special attention to their students/children/wards academic performances in English composition writing and engage them with frequent exercises with prompt feedback for improved academic performance.