Background: Children with motor delays are at increased risk for delayed means-end problem-solving (MEPS) performance. Objectives: To evaluate children with motor delays: 1) the impact of motor delay severity and MEPS mastery timing on developmental trajectories of MEPS; and 2) the effectiveness of Sitting Together And Reaching To Play (START-Play) intervention for improving MEPS. Methods: This represents a secondary analysis from a multi-site randomized controlled trial, with blinded assessors and prospective registration. Children with mild or significant motor delays (n=112, Mean=10.80, SD=2.59 months at baseline) were randomly assigned to START-Play or usual care early intervention (UC-EI) and assessed at five visits across one year using the Means-End Problem-Solving Assessment Tool that included three 30-second MEPS trials per visit. Task mastery occurred at the first visit the child achieved the highest level of performance in at least two of the three trials. Multilevel analyses evaluated trajectories of MEPS outcomes dependent upon the timing of MEPS mastery, motor delay severity, and intervention group. Results: At baseline, children with mild motor delays demonstrated better MEPS than children with significant delays, but this difference was only observed for children who achieved mastery late. Children with significant delays demonstrated greater improvements in MEPS in the post-intervention phase compared to children with mild delays. No MEPS differences were found between START-Play and UC-EI. Conclusion: Motor delay severity and timing of task mastery impacted MEPS trajectories, whereas START-Play intervention did not impact MEPS for children with motor delays.