With longitudinal survey design of survey research method, this study investigated the complex relationships between intelligence and academic achievement in Mathematics and English Language over a period of four years in Nigeria. A random sample of 637 Junior Secondary III students, aged 14 years, was drawn and followed till the end of their Senior Secondary III at the age of 17. Their Mathematics and English Language academic achievement were validly measured with Junior Secondary Certificate Examination (JSCE) scores and Senior Secondary Certificate Examination (SSCE) scores. Their IQ was validly and reliably measured with Culture Fair Intelligence Test that has been validated and standardized for use in Nigeria. Data were collected during the students’ 2011/2012 JSCE and 2014/2015 SSCE. The IQ and JSCE as well as SSCE scores were subjected to partial correlation analysis at 0.05 alpha, using SPSS Version 22. Results showed statistically significant relationship between IQ and Mathematics achievement (0.499 and 0.495) when English Language is partialled out; and between IQ and English Language achievement (0.411 and 0.346) when Mathematics is partialled out; respectively across the junior and senior secondary levels of schooling. Results, among others, further indicated overwhelming evidence of stability of intelligence (0.702) with the four-year time interval in super corroboration of fluid and crystalized theory of intelligence. Coefficient of partial determination unveiled that IQ accounts for 24.90% to 24.50% of the variance in Mathematics achievement, and 16.89% to 11.97% of the variance in English Language achievement.