Purpose: This study examined the production of morphosyntactic markers by school-age children with and without developmental language disorder. Comparisons were made between students who speak mainstream American English (MAE) dialects and nonmainstream American English (NMAE) dialects. Method: First- and second-grade students (N = 82) completed assessments of dialect use and language ability, which are designed for students who speak NMAE dialects. Students also completed an experimental production task targeting three morphosyntactic features: past tense "-ed" marking, third-person singular "-s" marking, and plural "-s" marking. Past tense marking and third-person singular are produced differently across MAE and NMAE dialects, whereas plural marking is produced more similarly across dialects. Results: When comparing across dialects, children with typical language skills who spoke NMAE dialects overtly marked past tense and third-person singular less often compared to MAE peers. However, when comparing to same-dialect peers with language disorders, children with typical language skills who spoke NMAE dialects overtly marked these morphosyntactic markers more often than peers with developmental language disorder. Conclusion: The results underscore the importance of considering a child's dialect use when assessing language ability, in particular with measures that include features that are variable in NMAE dialects. At the same time, within-dialect comparisons suggest that a broader set of morphosyntactic features may provide useful information for evaluations of language ability. Future research should investigate the source of these differences, including the extent to which students with language disorders have acquired the social and linguistic factors that condition the use of variable features.