IMPORTANCE: The extent of mother's milk provision by race and ethnicity and its change over time is poorly understood in the US preterm population. OBJECTIVE: To examine US prevalence and trends over time of mother's milk initiation and continuation at 12 weeks after birth (2009-2019) by maternal race and ethnicity and to examine associations of maternal race and ethnicity and mother's milk initiation and continuation among mothers of preterm infants. DESIGN, SETTING, AND PARTICIPANTS: This cross-sectional study used data from the nationally representative Pregnancy Risk Assessment Monitoring System database. Participants included mothers of preterm infants identifying as non-Hispanic Asian, non-Hispanic Black, Hispanic any race, or non-Hispanic White, with survey completion 12 or more weeks post partum from 2009 to 2019. Data were analyzed from February 2022 to June 2024. EXPOSURE: Maternal race and ethnicity obtained from birth certificate data. MAIN OUTCOMES AND MEASURES: Mother's milk initiation and continuation at 12 weeks were assessed using survey questions. The overall prevalence and trends over time by maternal race and ethnicity were determined. Multivariable regression was used to analyze independent associations between maternal race and ethnicity and mother's milk initiation and continuation. RESULTS: Among 1 523 131 (weighted) mother-preterm infant dyads, mother's milk initiation increased significantly from 2009 to 2019 for White and Black mothers but not for Asian or Hispanic mothers. Mother's milk provision at 12 weeks increased significantly among all groups. There were significant differences in mother's milk outcomes between groups. Initiation was highest for Asian mothers (92.8%; 95% CI, 91.1%-94.4%), followed by Hispanic (88.1%; 95% CI, 86.5%-89.8%), White (84.1%; 95% CI, 83.3%-84.9%), and Black (75.3%; 95% CI, 73.4%-76.6%) mothers. After adjusting for covariates of interest, compared with White mothers, initiation remained higher for Asian (adjusted relative risk [aRR], 1.09; 95% CI, 1.06-1.12) and Hispanic (aRR, 1.10; 95% CI, 1.08-1.12) mothers. Continuation at 12 weeks was highest for Asian mothers (65.4%; 95% CI, 62.7%-68.0%), followed by Hispanic (48.2%; 95% CI, 46.1%-50.3%), White (47.7%; 95% CI, 46.7%-48.7%), and Black (34.3%; 95% CI, 32.9%-35.7%) mothers. After adjustment, continuation was higher for Asian (aRR, 1.37; 95% CI, 1.24-1.47) and Hispanic (aRR, 1.33; 95% CI, 1.27-1.41) mothers compared with White mothers. CONCLUSIONS AND RELEVANCE: In this cross-sectional study, mother's milk provision among preterm infants increased from 2009 to 2019, but notable racial and ethnic disparities persist. Future work should address barriers to continued provision of mother's milk for preterm infants.