While it is known that intrauterine growth restriction is associated with later mental disorders, it is still unclear whether similar associations exists for postnatal weight faltering, also known as 'failure to thrive' in infancy. This study examined the potential connection between infancy weight faltering and mental disorders diagnosed in childhood focusing specifically on neurodevelopmental disorders. The Copenhagen Child Cohort (CCC2000) was used to explore weight gain in infancy assessed by community health nurses. Data from the Danish national registries were used to quantify ICD-10 mental disorders diagnosed between birth and 12 years of age, as well as potential child and family confounders. Of 4.476 children with sufficient weight data, 339 (7.3%) children were diagnosed with a mental disorder in childhood. Both any (weight gain < -1SD) and severe infancy weight faltering (weight gain < -2SD) were associated with psychomotor delays, while severe infancy weight faltering was also associated with intellectual impairments. Notably, no significant associations were found between weight faltering and autism spectrum disorders or attention deficit hyperactivity disorders. Weight faltering in infancy may be an early marker of neurodevelopmental delays. This possibility should be considered when assessing infants with slow weight gain, to early identification and treatment of co-occurring neurodevelopmental disorders.