Due to the importance of teachers in providing quality education, research in the last two decades has consistently focused on their effectiveness. However, there are inconclusive debates in the literature on the nature of relationships that exist between different supervisory strategies and their links to teachers' job performance. This study used a predictive correlation design to assess how five principals' supervisory practices contribute to teachers' job effectiveness generally and across three specific areas. Secondary school teachers that participated in the study were 284 and were selected using simple and purposive sampling techniques. Data were collected using a questionnaire. A bootstrapping procedure in covariance-based structural equation modelling (CB-SEM) and multiple linear regression analysis were used for hypotheses testing at the .05 alpha level. Among others, findings revealed a significant joint prediction of classroom observation, supervision of sporting activities, instructional aids, team teaching and school record on teachers' job effectiveness generally and in the dimensions of lesson note preparation and classroom management, but not the use of instructional materials. Teachers who reported frequent classroom observation by principals also reported a higher level of lesson preparation but not other aspects of teaching effectiveness. The supervision of sporting activities significantly predicted teachers' overall effectiveness and specific aspects such as lesson preparation, instructional material use, and classroom management. This study can encourage school principals to gain more knowledge, facts and strategies required for effective and efficient school-based supervision.