Extension service plays a crucial role in agricultural production and household food security in Africa south of the Sahara (SSA). Despite the growing evidence on the effects of extension service on various outcomes, including crop productivity, technology adoption, and income, the evidence is limited on the impact on farmers’ ability to select and allocate inputs. While the effects of the extension service can vary with spatial variation, our study examines the effects of extension crop (maize, bean, and rice) production, revenue, and profit in Mbale district, located in the Eastern region of Uganda. We decompose the effects of extension into three: worker effect, input-allocation effect, and input-selection effect. Using household-level data based on the 204 farmers' surveys, we examine the effects of extension service. Both worker and allocative effects of rice are found, while the former is more dominant than the latter. The results also show that the input-selection effect is more influential than the input-allocation effect. Overall, the findings suggest that much attention should be paid to rice farmers in Mbale district by supporting access to agricultural and market information with innovations in farm technology that enable farmers to receive optimal returns in crop production. Given the effects of extension service, tailored strategies and agricultural programs are suggested to enhance the capacity of farmers and increase profitable marketed output that will help to promote sustainable development.