Dairy cooperatives play a critical role in integrating smallholder farmers into milk markets by aggregating production, reducing transaction costs, strengthening collective bargaining power, and facilitating access to markets and services. However, their operational performance may be constrained by dispersed milk production, inefficient collection and transportation systems, inadequate infrastructure, and weak market coordination. This study examined the association between aggregation strategies and the operational performance of dairy cooperative societies in Kiambu County, Kenya. A descriptive cross-sectional research design was employed, covering 19 dairy cooperative societies. Managers, board members, and cooperative-member employees served as key informants. A stratified purposive sampling procedure targeted 78 respondents, of whom 69 completed the questionnaire, yielding an 88.46% response rate. Primary data were collected using structured questionnaires, while secondary data on 2024 milk production volumes and milk payment rates were obtained from the Kenya Dairy Board. Operational performance was measured using a composite Operational Performance Index (OPI), calculated as annual milk volume multiplied by the average annual milk payment per litre and subsequently natural-log transformed. Data were analysed using descriptive statistics, correlation analysis, diagnostic tests, and multiple linear regression. The findings revealed a strong positive correlation between aggregation strategies and operational performance (r = 0.852, p < 0.01). After controlling for technology adoption and governance practices, aggregation strategies remained a positive and statistically significant predictor of operational performance (β = 0.988, SE = 0.346, t = 2.86, p = 0.012). The overall regression model was statistically significant and explained 85.7% of the variation in log operational performance (R² = 0.857; F(3,15) = 29.874, p < 0.001). Qualitative findings identified decentralized milk collection, investment in collection and cooling infrastructure, collective marketing, coordinated transportation, and cost-efficient aggregation as key mechanisms through which cooperatives could improve operational efficiency and market coordination. The study concludes that well-organized aggregation strategies are an important organizational capability associated with stronger operational performance among dairy cooperatives. It recommends strategic investment in decentralized collection systems, cold-chain infrastructure, coordinated transportation, collective marketing, and cost-management practices, complemented by effective technology adoption and governance. Given the cross-sectional design and small number of cooperatives, the findings should be interpreted as evidence of association rather than definitive causality.