Few studies have investigated associations of milestone development in early childhood with intelligence in adulthood in typically developing children. The current study is an extension of 2 previous studies on smaller samples and investigated associations of age at attainment of 32 developmental milestones attained between 0 and 3 years of age with adult intelligence and explored whether the effects of early infant milestones are mediated through later development during subsequent years. Mothers of 8,400 children from the Copenhagen Perinatal Cohort recorded 32 developmental milestones during the child's first 3 years of life. Information on at least 1 developmental milestone was available for 2 subsamples with adult follow-up information on intelligence: Børge Priens Prøve (BPP) was available for 2,567 men while Wechsler Adult Intelligence Scale (WAIS) was available for 1,000 men and women. The study found that early attainment of milestones, primarily those related to standing/walking and language development, was associated with higher intelligence in adulthood. The adjusted BPP means were 103.7, 101.7, and 99.5, respectively, for being able to name objects/animals in pictures at less than 18 months, 18-24 months, and later than 24 months. Mediation analyses showed direct associations of the 1-year milestones related to standing/walking with intelligence with the direct association accounting for 74.6% and 64.4% of the total association in each subsample. Thus, milestones related to standing and walking primarily show a direct association with adult intelligence and are to a smaller extent mediated by milestones reflecting development during the subsequent years, in particular language development.