This study investigates the role principal supervision on the relationship between students’ personnel services and academic achievement in secondary school via mixed-method approaches (quantitative and qualitative). Sample size of 100 students in one of the secondary schools formed the respondents for the quantitative study, while principals (junior and senior section) of the selected school formed the informants for the qualitative study. Questionnaire tagged “Students’ Personnel Services and Supervision Questionnaire (SPSPQ)” was used to elicit data from students while interview protocol tagged “Principal Supervision, Students’ Personnel Services and Academic Achievement (PSA)” was used to collect relevant data from the principals. Data collected through quantitative study was analysed using Statistical Package for Social Sciences (SPSS) and Partial Least Square (PLS software) were used to assess the psychometric properties of the constructs, an Nvivo software (version 10) was used for coding and thematic analysis of the data collected. Quantitative findings revealed that the individual item reliability of the constructs had loadings between .709 and .956 while the composite reliability coefficients of the latent constructs had loadings between .81 and .98. Similarly, the average variance explained of showed high loading (minimum of .68 and maxmum of .78 ) on their respective constructs. In addition, qualitative findings showed that services are provided for students in secondary schools. Conclusively, the preliminary results from the two studies are reliable , therefore it demonstrates the potency for the conduct of the main study on the role of principal supervision on the relationship between students’ personnel services and academic achievement in secondary schools.