Fathers and grandmothers are key family influencers who have an impact on maternal and child health. This paper describes the planning, design, and implementation of a four-phased evaluation study on the impact of engaging fathers or grandmothers in improving diets of mothers and feeding practices of infants and young children in a rural setting in western Kenya. The study used a quasi-experimental, non-equivalent comparison group design with pre- and post-test observations. It tested the hypothesis that families participating in activities to engage fathers or grandmothers have better knowledge and adopt better practices related to maternal nutrition and complementary feeding than families for which nutrition messages are targeted only to mothers. Information generated from previous formative research was used to design culturally relevant interventions for fathers and grandmothers. Interventions included separate but parallel peer education dialogue groups with fathers and grandmothers. They were held twice a month, and family bazaars and special fathers’ days at local clinics were held once per month. The study team selected the dialogue-based group methodology, including promotion of social support actions, because it engages participants to actively discuss new information and experiences providing social support rather than passively receive information. Community health extension workers and Ministry of Health nutritionists provided supportive supervision and monitoring of the dialogue group activities. Father and grandmother interventions were implemented in two separate sub-locations of Kenya’s former Western Province, and the program effects were compared to findings in a population with a similar socioeconomic background living in a similar sub-location in the same province. An examination of the study implementation methodology provides useful insights into practical issues that need to be addressed in programs seeking to engage key household influencers of maternal nutrition and infant and young child feeding behaviours. Results showed that the dialogue group methodology, formative research to inform intervention design, use of the existing Ministry of Health community health unit structure to provide critical support supervision, and actions that motivated pee mentors were key factors for successful implementation of the study intervention.