This study sought to assess supply chain rationalization practices on performance of large manufacturing firms in Nairobi City County, Kenya. In specific, the study aimed to examine the effect of sourcing practices, information visibility, strategic relationship and to explore the effect of demand planning on performance of large manufacturing firms. This study was built on Transaction Cost Economics Theory, Network Theory, Relational Exchange Theory and Dynamic Capability Theory. The researcher adopted a descriptive research design. The target population was observed from the 499 supply chain managers and analyzed in manufacturing firms in Nairobi City County. A stratified random sampling technique was employed in attaining a representative sample from the target population to obtain a sample size of 222 respondents using formula proposed by Yamane. A closed-ended questionnaire was used to collect raw data from the respondents and analyzed using Statistical Package for Social Sciences version 25 to compute percentages of respondents' answers. Inferential statistics using multiple linear regression was applied to assist examine relationship between research variables in the study. The study found out that supply chain rationalization practices had a positive significant influence on performance with coefficient determination of 0.785 signifying that supply chain rationalization practices; sourcing practices, information visibility, strategic relationship and demand planning, contributed to 78.5% in performance of large manufacturing firms in Nairobi City County, Kenya. The study therefore concluded that when these supply chain rationalization practices are properly implemented, then improved performance would be realized. The study recommended for an establishment of proper organization's structure and a holistic adoption of supply chain rationalization practices in large manufacturing firms in Nairobi City County by all levels of management.