Background The prevalence of autism-spectrum disorder (ASD) has been shown to be higher in migrant families, but it is also a challenge for health care professionals to offer adequate services to families that face multiple challenges. In the context of the EPIGRAM study (a French prospective, multisite, longitudinal observational study implementing integrative care practices (ICPs) for children with ASD), we aimed to assess the impact of migration on children with ASD. Method and findings 89 children with ASD aged 3 to 6 years old (92% males) were recruited and followed up for 12 months. The children were clinically assessed using several instruments. At baseline, children had severe autism on average on the Children Autism Rating Scale (CARS, mean = 44; SD = 6.51) and moderate autism on the PsychoEducational profile-3-R (PEP-3-R) maladaptive behavior category (mean = 30; SD = 29.89). Thirty percent of the families had a low socio-economic status, and 56% were first-generation immigrants. For all clinical variables, children of immigrant parents had more severe autism and developmental delays at baseline. A linear mixed model established an improvement in all clinical characteristics over the 12 months of the study. This trend may be attributed to ICPs or any naturally occurring event during that period. Families shared this positive view over time. However, the improvements were slower for two clinical dimensions of the PEP-3-R in children from migrant families. For the inappropriate behavior category, the time effect diminished by an average of 0.83 percentile/month for children whose parents were migrants vs. children whose parents were non-migrants. Similarly, for verbal behavior characteristics, the time effect diminished by an average of 1.32 percentile/month for children whose parents were migrants vs. children whose parents were non-migrants. Conclusion Despite an overall positive improvement, we found that migration is associated baseline severity and progress over time in children with ASD. There is an urgent need to target the migrant population with specific research and understand the avenues that carry such higher severity.