Importance: Maternal attention-deficit disorder with or without hyperactivity (ADHD) is an established risk factor for adverse perinatal outcomes that remains understudied to breastfeeding. Objective: To estimate the effect of maternal ADHD on 6-month exclusive breastfeeding. Design: A prospective longitudinal community-based cohort study. Setting: Canada, 2014-2020. Participants: We included 3217 English-literate mother-father dyads with internet or telephone access, comprising women with singleton birth within 3 weeks postpartum (study baseline) and were cohabiting with the biological father. Women with major psychiatric conditions or suicidality were excluded. All participants provided informed consent at recruitment. Data were collected online through self-report questionnaires. Exposure: Maternal ADHD was measured at baseline using the Adult ADHD Self-Report Scale (sum of scores [≥]14). Main Outcomes and Measures: We defined 6-month exclusive breastfeeding based on maternal reported infant feeding, assessed at 3 and 6 months. Maternal childhood adversity (Adverse Childhood Experiences International Questionnaire, ACE-IQ; sum score) was included as a confounder. The average treatment effect on the treated (ATT) was estimated using linear probability models. We calculated 95% percentile bootstrap CI. Missing data were handled using multiple imputation by chained equations with 10 iterations. Results: Complete-case analyses included 2071/3217 mother-father dyads, most of which were Canadian-born (79%). Childhood adversity, mental health conditions, and substance use were higher among mothers who screened positive for ADHD (4%) than among those who did not: the median number of ACE was 5 IQR 4-6; 38% self-reported prenatal depression, 13% smoked cigarettes and 19% drank alcohol during pregnancy; 11% of infants used the neonatal intensive care unit. Notably, though 90% of mothers with ADHD intended to breastfeed exclusively only 28% breastfed exclusively for 6 months. Among families with maternal ADHD, the effect of maternal ADHD on 6-month exclusive breastfeeding was ATT -0.105, 95% CI -0.209 to -0.003 in the model adjusted for maternal ACE and paternal ADHD. With multiple imputation: ATT -0.109, 95% CI -0.213 to -0.004. Conclusions and Relevance: Mothers with ADHD intend to breastfeed their infants exclusively, but additional challenges shorten the duration of exclusive breastfeeding, indicating a need for interventions tailored to neurodevelopmental differences.