The purpose of this study was to determine the influence of gender stereotypes on the effectiveness of women senior managers in Non-Governmental Organizations (NGOs) in Nakuru County, Kenya. A descriptive survey design was used and the target population was the human resource managers and women senior managers in 65 duly registered and active NGOs in Nakuru County. From these organizations, simple random sampling was used to select a human resource manager and a woman senior manager which resulted in 130 respondents. Primary data was collected using a self-administered questionnaire which had close-ended and open-ended questions while secondary data was collected through the use of document analysis. Quantitative data was sorted, cleaned and analyzed through the SPSS software. Descriptive statistics were analyzed through the use of frequencies and percentages while inferential statistics were analyzed through the use of Pearson Correlation and Pearson Chi-Square tests. The findings of the analysis were presented in form of tables and charts. On the other hand, qualitative data were analyzed thematically and the results were presented in the form of narratives. The study adhered to and ensured that research ethics were upheld. The study found out that gender stereotypes had a statistically significant negative relationship with the effectiveness of women senior managers (r= -0.342; p<0.05). The Chi-square test results indicated that gender stereotypes had a statistically significant association with the effectiveness of women senior managers. The recommendation made was that a similar study can be conducted to provide both the perspectives of men and women senior managers.