Despite growing evidence that supports the importance of exclusive breastfeeding of infants below six months, few mothers adhere to this. Early weaning to infant formula and solids is a common practice. This study aimed to ascertain the determinants of breastfeeding practices among mothers at maternal and child health clinics at Pumwani hospital in Nairobi County, Kenya. The theory of reasoned action guides this study. It was designed as an institutional-based cross-sectional analytical study. A total of 307 mothers and 12 community health workers were sampled to participate in the study. Both qualitative and quantitative data collection tools were administered to the sampled respondents. Data analysis was conducted using both qualitative and quantitative methods. All data are presented in tables, graphs, and verbatim. Throughout the study period, strict adherence to ethical principles was strictly observed. The study established that 67% of the mothers-initiated breastfeeding within the first hour of delivery. However, only 37% of the mothers practiced exclusive breastfeeding. Demographic factors established to be influencing exclusive breastfeeding included the age of both the mother and the father, levels of education for both the mother and the father, religion, and marital status (p < 0.05). Family size did not influence exclusive breastfeeding practices (p > 0.05). The study also established that up to 83% of the mothers had high levels of knowledge on exclusive breastfeeding practices. However, this knowledge did not translate to practice as there was no significant statistical relationship between knowledge and practice (p > 0.05). The study also established that most mothers (69.2%) had a negative attitude towards exclusive breastfeeding. This attitude was found to have a significant statistical association with exclusive breastfeeding practices (p < 0.05). Therefore, recommendations are made to develop county-specific policy amendments to ensure adequate facilitation in the health facilities towards the realization of exclusive breastfeeding requirements. Awareness drives could also be organized in the study area to address negative attitudes and their causes in the study area.