Purpose: Children with Down syndrome (DS) may have limited opportunities to engage in independent mobility at the same age as their typically developing peers due to motor delays. The primary purpose of this study was to examine two activity-oriented outcomes - (1) onset of independent driving, and (2) onset of independent walking – in relation to total dosage and daily usage of a modified ride-on car during a progressive, 7-16 month intervention for young children with DS. The secondary purpose of this study was to describe the sample’s age of onset for select motor milestones in comparison to DS norms. Method: Eight young children with DS participated in this home-based intervention. Two different models of the modified ride-on car were used: a seated modified ride-on car and a sit-to-stand (STS) modified ride-on car, in which the child had to pull-to-stand and remain standing in order to power the vehicle. The STS car could also be used as a power-push device. Results: All eight children were able to independently drive the modified ride-on car in both seated and standing modes, with 5 of the 8 families electing to use the STS car as a power-push device. Further, children in this sample achieved motor milestones earlier than would be expected for children with Down syndrome, with 6 of the 8 walking before 24-months. Conclusions: Although there was no distinct relation between dosage and outcomes, intricacies of modified ride-on car usage patterns suggest that further investigation of the dose-response relation is warranted. Results from this study inform future larger scale studies by highlighting dose-response relations of modified ride-on car interventions and developmental outcomes among young children with DS.