Abstract Farmer groups are important socioeconomic safety nets for rural communities in sub-Saharan Africa. They provide mutual support to farmers through collective action to enhance improvement of livelihoods. These groups have been increasing in number in the post liberalization period in East Africa. However, it is not clear how these groups’ organizational arrangements have been changing over time, and the contribution of these changes on effectiveness of collective action. This study, therefore investigates the various organizational changes in groups and how these changes influence effectiveness of collective action of farmer groups in East Africa. Data were collected from 195 farmer groups in Kenya and Uganda through a structured questionnaire survey, supplemented by focus group discussions. Results show that changes in leadership and governance structures were more important in influencing effectiveness of groups. These include having a considerable number of leadership positions, introducing clear leadership tenure, having committees in groups, holding frequent committee meetings and putting in place more enforcement mechanisms for regulating group conduct. This study proposes adoption of effective leadership and governance structures by farmer groups to enhance effectiveness of collective action. Key words: Organizational structures, organizational changes, small-holder farmers, farmer groups, group objectives