This paper investigated the influence of income levels on dairy farmers’ membership in cooperative societies, focusing on the Githunguri Dairy Farmers Cooperative Society in Kiambu County, Kenya. Cooperative societies are widely recognized for promoting economic stability, collective bargaining, and market access among smallholder farmers. While previous studies have shown that cooperative membership can enhance household income, there is limited empirical evidence on how pre-existing income levels determine farmers’ decisions to join and remain active in cooperatives. Using a descriptive research design, data were collected from 316 respondents comprising management and staff through structured questionnaires. Descriptive statistics summarized member perceptions of income stability, while inferential analyses—including Pearson’s correlation and simple linear regression—examined the relationship between income levels and membership. The findings indicate a positive and significant correlation (r = 0.523, p < 0.01), with regression analysis confirming that income levels significantly predict cooperative membership (β = 0.215, p < 0.01), explaining 27.3% of the variation in membership. Higher and more stable income enhances farmers’ ability to meet cooperative obligations, strengthens trust in management, and fosters long-term participation. These results support Rational Choice Theory by demonstrating that farmers’ economic incentives influence their cooperative engagement. Practically, the study recommends implementing stable pricing, timely bonuses, and transparent profitsharing mechanisms to enhance member loyalty, while policymakers should support initiatives that stabilize incomes and reduce barriers to participation. Future research could explore the impact of digital platforms and socio-cultural networks on membership decisions, complementing income-based analyses.