Purpose: To investigate ophthalmic onset manifestations and the impact of diagnostic delay on the prognosis in infants (<1 year) diagnosed with a brain tumour. Methods: A retrospective population-based nationwide study of infants diagnosed with a brain tumour between 2007 and 2017 in Denmark. Data was retrieved from the Danish Childhood Cancer Registry, the National Danish Health registries, and medical files. Primary outcome measures included symptoms, clinical findings, time to diagnosis and survival. Results: Thirty-seven infants were diagnosed with a brain tumour in Denmark between 2007 and 2017. In total, 19/37 infants (51%, 95% CI: 34–68) had ophthalmic manifestations at any time prior to or at diagnosis; and in 6/37 (16%, 95% CI: 6–32) ophthalmic manifestations were the initial symptom. The most common ophthalmic manifestations were strabismus (n = 7), sunset eyes (n = 6), nystagmus (n = 4), reduced pupillary light reflex (n = 4), and/or decreased vision (n = 4). The median number of symptoms per infant at the time of diagnosis was three (range 0–9). The median diagnostic delay was 26 days (range 0–283, IQR: 6;90). 5-year survival rate was 75% (95% CI: 61–90) and all children with diagnostic delay > 100 days (n = 9, 24%) were still alive at the end of follow-up (median 6.3 years, range 2.2–10.2). Conclusion: We provide an overview of symptoms and clinical signs in a nation-wide series of infants with CNS tumours and demonstrate that ophthalmic manifestations are frequently observed in infants prior to diagnosis, but, often in combination with other clinical signs. The diagnostic delay was substantial for a large part of the infants, but this was not associated with increased mortality.