The present register-based study investigated the role of IQ in predicting a wide range of indicators of unsuccessful educational and occupational achievement among young men born across five decades in Denmark. The study population comprised all men who have been born since 1950 and have appeared before a draft board during the periods from 1968 to 1984 and from 1987 to 2015 (N = 1,098,742). IQ was assessed by Børge Priens Prøve at age 18. Unsuccessful educational achievement was indicated by leaving lower secondary school without a certificate, by no completed youth education at age 25, by no completed education leading to vocational qualifications at age 30, and by the total number of interruptions to education at age 30. Unsuccessful occupational achievement was indicated by not being in employment, education or training at age 30, by unemployment at age 30, by receiving sickness benefits at age 30, by receiving welfare benefits at age 30, by receiving disability pension at age 30, and by gross income at age 30. Binary logistic regression, negative binomial regression and median regression were used to estimate the associations of IQ with unsuccessful educational and occupational achievement. The results showed that low IQ was a strong and consistent predictor of all indicators of unsuccessful educational and occupational achievement. In conclusion, the study findings suggest that assessment of intelligence may provide crucial information for educational planning and counselling of poor-functioning schoolchildren and adolescents with regard to both the immediate educational goals and the more distant work-related future.