To determine the effect of time waiting for admission to inpatient neurorehabilitation following acquired brain injury on rehabilitation outcomes. A retrospective observational case series. A specialist brain injury inpatient rehabilitation service. Consecutive 235 admissions to specialist brain injury rehabilitation following acutely-acquired brain injury between 2019 and 2022. Waiting time from the point of injury to admission, diagnostic category, admission complexity (patient categorisation tool), functional status (functional independence measure/functional attainment measure), care needs (Northwick Park Care Needs Assessment), change in functional status and care needs over duration of admission (efficiency). Subgroup analysis was performed for patients with a tracheostomy, enteral feeding, anticonvulsant treatment and prior neurosurgery. There was no relationship between admission wait and initial complexity (rs = 0.006; p = 0.923), functional status (rs = -0.070; p = 0.284) or care needs (rs = 0.019; p = 0.768). Longer waiting times were significantly associated with reduced efficiency of rehabilitation (rs = -0.240; p = 0.0002) and change of care needs (rs = -0.246; p = 0.0001). Longer waits were associated with reduced rehabilitation efficiency for patients prescribed anticonvulsants (n = 115; rs = -0.243; p = 0.009), with a tracheostomy (n = 46; rs = -0.362; p = 0.013), requiring enteral nutrition (n = 137; rs = -0.237; p = 0.005) or having had intracranial surgery (n = 97; rs = -0.344; p = 0.0006). There was a negative association between waiting times and reduction in care needs for patients admitted on anticonvulsants (rs = -0.319; p = 0.0005) and requiring enteral nutrition (rs = -0.269; p = 0.001). Longer wait for transfer to rehabilitation following brain injury is associated with reduced improvement in functional status and care needs over time. Attention should be given to ensuring rapid transfer into inpatient rehabilitation services.