Research emphasizes the importance of seamless transitions from Early Behavioral Intervention (EBI) to school-based services, but formal support during this critical period remains limited, leading to decreased parental satisfaction with autism services. This study aimed to systematically document parents’ perception of the quality of services received during the transition from EBI to kindergarten, along with the predictors of this perception. The study adopted a prospective longitudinal design and relied on a validated evaluation framework, the Evaluation of the Trajectory in Autism for Parents (ETAP). ETAP allows to investigate five critical determinants (accessibility, continuity, validity, flexibility, empathy) of service quality, as rated by parents (N = 138 families), which were evaluated at three critical periods of the transition: end of EBI, beginning and end of first year of school. Quality ratings indicated generally positive perceptions but showed small to medium effect size decreases over time; validity had the largest decrease. Predictors of quality ratings and changes in ratings differed by phase of the transition and included child characteristics (gender, age, disabilities, challenging behavior), parent characteristics (maternal origin, fluency in the official language of the province, education level), and family characteristics (number of children). This study’s comprehensive analysis of service quality determinants and possible risk and protective factors provides insights into improving support for families navigating the transition to school.