This study explored gross motor development (GMD) trajectories among 6359 children, with and without autism, from the Growing Up in New Zealand longitudinal cohort study. By the age of 8, 173 children had either an autism diagnosis (n = 108) or parent-reported autism concerns (n = 65). Gross motor milestones were reported by mothers when children were 9, 24, and 54 months of age. We found that irrespective of autism diagnosis, GMD delays at 24 months of age were more likely among girls, children born preterm, and those whose mothers identified as European. A mixed-effect logistic regression model, controlling for antenatal maternal and child covariates, revealed that the proportion of children with GMD delay (relative to their peers) increased significantly from 9 to 54 months for all three groups, but the increase was greater for those with autism concerns (OR = 1.28, 95% CI = 1.08-1.52) or an autism diagnosis (OR = 1.26, 95% CI = 1.10-1.43) compared to the no autism group (OR = 1.06, 95% CI = 1.02-1.10). Differences in the changes in GMD performance among children with an autism diagnosis compared to those without autism occurred between 9 and 24 months (OR = 2.16, 95% CI = 1.13-4.13). No significant GMD delay differences were found at any time between children with an autism diagnosis versus those with autism concerns. Children with a GMD delay should be screened for autism at 24 m. Early identification is the first step toward knowledge-based, effective intervention of developmental difficulties.