Manufacturing firms operating in the presents' rapidly changing and highly competitive market have been pressured to improve all aspects of operational performance; quality, flexibility and customer response time as well as reduce costs of manufacturing. They have retorted by adopting a set of practices that is fast becoming the dominant paradigm in manufacturing - lean manufacturing. Lean manufacturing involves continuous elimination of waste from the value chain of manufacturers thus enhancing customer value through continuous improvement of operational performance of the manufacturers. This research sought to determine the effect of lean manufacturing practices on the operational performance of manufacturing firms in Mombasa County. The study was guided by to three objectives: To determine the extent to which manufacturing firms in Mombasa County have adopted lean manufacturing practices; To establish the effect of lean manufacturing practices on operational performance of manufacturing firms in Mombasa County and finally the challenges faced by firms when implementing lean manufacturing. The data analyzed was gathered using a semi-structured questionnaire targeting operations managers of manufacturing firms in Mombasa. The results were presented using tables, pie charts and were analyzed using descriptive statistics and regression models. The findings indicated that most manufacturing firms in Mombasa practiced lean manufacturing. It was also clear that lean manufacturing firms had seen an improvement in their operational performance. The effect of lean manufacturing practices on the operational performance of manufacturing firms was tested against four operational performance metrics; quality, flexibility, speed and cost). The outcomes showed that lean manufacturing practices; just-in-time, continuous improvement/kaizen, value stream mapping and total productive maintenance; are positively related to operational performance while automation is negatively related to operational performance. The study also established that the most experienced challenge in adopting lean manufacturing was high costs of implementation and the least was lack of top management support. The study recommends more awareness of the importance of lean manufacturing practices in manufacturing firms and support from the top management as critical tool for takeoff. Most of all, the researcher urges manufacturing firms to implement lean manufacturing practices and create a culture that accommodates them since they have excellent systems and structures that can productively support it. If lean manufacturing practices are consistently implemented the operational performance of the manufacturing firms can significantly improve and this will enable manufacturing firms to survive the harsh business environment. The study also recommends that for lean manufacturing to be successful it should to be practiced within the entire supply chain.