The insurance companies constitute a fundamental building block of the global financial system that provides opportunities for hedging against an assortment of risks and as well serve as institutional investors thus promoting sustainability and growth of national economies. Despite the significant role played by the insurance industry in supporting the national economy, there has been a notable decrease in insurance penetration trend. Similarly, the insurance industry has registered an increase in the number of complaints lodged by customers in relation to delayed settlement, erroneous deductions, and unsatisfactory offers. This study therefore sought to investigate the effect of assimilation capacity on performance of insurance companies in Nairobi City County, Kenya. The study was grounded on systems theory and dynamic capabilities theory. Explanatory research design were adopted in this study. The target population of this study comprised of 59 insurance companies operating in Nairobi City County. Cross-sectional data were collected using a semi-structured questionnaire from 216 heads of functional areas in 27 insurance companies which were selected using proportionate stratified random and simple random sampling. Face, construct and content validity of the research instrument were confirmed accordingly. A pilot study was conducted to aid in the statistical test of reliability using Cronbach alpha index of at least 0.7. Administration of the questionnaire was done using drop-and-pick later method. The study had a response rate of 81 percent. Quantitative data were analyzed using descriptive and inferential statistics. Descriptive analysis involved the use of frequency count, percentages, sample mean, sample standard deviation and coefficient of variation. Test of assumptions of linear regression analysis was performed. Linear regression analysis was used to estimate the population parameters and facilitate testing of hypotheses at 95 percent level of confidence. Results of data analysis were presented in tabular form as well as using figures. Qualitative data were analysed using content analysis. The study found out that assimilation capacity has a positive effect on organizational performance. The human resources managers should provide policy guidelines in support of activities that promote internalization of knowledge through simulation as this has potential to strengthen the contribution of assimilation capacity in insurance companies. Management of insurance companies should promote practices that enhance optimization of informational resources.