Purpose <p>Access to early autism intervention remains limited for many families, increasing interest in alternative service delivery models such as telehealth-based parent-mediated intervention. However, few studies have examined developmental trajectories observed under different service delivery formats within naturalistic clinical settings.</p> Methods <p>This longitudinal observational study examined developmental trajectories during a 20-session parent-mediated intervention delivered either in-person or through telehealth-based caregiver coaching for young autistic children. A total of 320 children were initially enrolled, and 297 were included in the final analytic sample (99 per group: in-person intervention, telehealth intervention, and typically developing comparison group). Developmental outcomes were assessed using the Ages and Stages Questionnaires (ASQ) and CDC developmental milestone indicators. Children in the intervention groups were assessed at baseline, Week 10, and Week 20. The typically developing group was assessed at baseline and Week 20 and was included for descriptive developmental context only. Linear mixed-effects models were used to examine developmental trajectories over time and to explore differences between service delivery formats.</p> Results <p>Across both intervention formats, caregiver-reported developmental scores increased over time on ASQ and CDC measures, with higher scores observed at Week 10 and Week 20 relative to baseline (ps &lt; 0.001). The typically developing comparison group maintained higher developmental scores throughout the study period and demonstrated expected age-related developmental progression over time. Although children in both ASD intervention groups demonstrated substantial developmental gains over 20 weeks, developmental functioning remained below that of the typically developing group at follow-up, indicating that the developmental gap narrowed but was not eliminated. Analyses did not detect statistically significant differences in developmental trajectories between in-person and telehealth delivery formats (ps ≥ 0.27).</p> Conclusions <p>Children participating in both in-person and telehealth parent-mediated intervention demonstrated increasing caregiver-reported developmental scores over time. Differences in developmental trajectories between service delivery formats were not detected within the precision of the present analyses. These findings should not be interpreted as evidence of equivalence or comparative effectiveness. Rather, they provide observational information regarding developmental trajectories under two real-world, family-centered service delivery models. Further research using randomized comparative designs, blinded outcome measures, standardized fidelity monitoring, and formal noninferiority or equivalence frameworks is needed to evaluate comparative effectiveness across delivery formats.</p>