Purpose: The aim of the study was to evaluate the efficacy of a theoretically motivated explicit intervention approach to improve regular past tense marking for early school-age children with developmental language disorder (DLD). Method: Twenty-one children with DLD (ages 5;9-6;9 [years;months]) were included in a crossover randomized controlled trial (intervention, n = 10; waiting control, n = 11). Intervention included once-weekly sessions over 10 weeks using the SHAPE CODING system, in combination with a systematic cueing hierarchy to teach past tense marking. Once the first group completed intervention, the waiting control group crossed over to the intervention condition. The primary outcome was criterion-referenced measures of past tense marking with standardized measures of expressive and receptive grammar as the secondary outcome. Ancillary analyses on extension and behavioral control measures of morphosyntax were also conducted. Results: There was a significant Time × Group interaction (p < 0.001) with a significant difference in pre--post intervention improvement in favor of the intervention group (p < 0.001, d = 3.03). Further analysis once both groups had received the intervention revealed no improvement for either group on past tense production during the 5-week pre-intervention period, significant improvement pre--post intervention (p < 0.001, d = 1.22), with gains maintained for 5 weeks postintervention. No significant differences were found on pre- to postintervention standardized measures of grammar, or on extension or control measures. Conclusions: The efficacy of the theoretically motivated explicit grammar intervention was demonstrated. Results contribute to the evidence base supporting this intervention to improve past tense production in early school-age children with DLD, suggesting it is a viable option for clinicians to select when treating morphosyntactic difficulties for this population.