The study aimed at investigating the principals’ leadership practice and instructional effectiveness of junior secondary schools in Ajeromi-Ifelodun local government of Lagos State, Nigeria. Surveys were conducted with 154 principals, 269 heads of the department and 25 members of staff from the Secondary Education Board in Niger State. The findings indicate that the extent of principal leadership practices and school effectiveness attributes in secondary schools in Lagos State is high. The result of multiple regressions shows that about 14% (R2 =0.14) of the variation in school effectiveness accounted for leadership practices. We recommend that the Federal and State Ministries of Education in Nigeria focus on issues other than leadership practices, such as training programmes for secondary school teachers to enhance school effectiveness. There is also a compelling need for secondary school principals to identify and promote the professional development needs of teachers and to ensure that teachers are effectively trained to enhance the development of secondary schools into centre of excellence.