Background: Due to shared health behaviors and disease risk, families may be more effective targets for health promotion. This study assessed whether providing family health history (FHH)-based risk information for heart disease and diabetes affected encouragement to engage in physical activity (PA) and healthy weight (HW) maintenance and co-engagement in physical activity among 320 Mexican-origin parents and their 1,081 children. Method: At baseline and 10 months, parents indicated who they encouraged and who encouraged them to engage in PA/HW, and with whom they co-engaged in PA. Households were randomized to receive FHH-based assessments either by one or all adult household members. Primary analyses consisted of regression analyses using generalized estimating equations. Results: At baseline, parents reported encouraging their child for both PA and HW in 37.6% of parent-child dyads and reported receiving children's encouragement for both in 12.1% of dyads. These increased to 56.8% and 17.5% at 10 months (p < 0.001). Co-engagement in PA increased from 11.4% to 15.7% (p < 0.001), with younger children (30.4%) and mother-daughter dyads (26.8%) most likely to co-engage at 10 months. Providing FHH-based risk information to all adult household members (vs. one) was associated with increased parent-to-child encouragement of PA/HW (p = 0.011) at 10 months but not child-to-parent encouragement. New encouragement from parent-to-child (p = 0.048) and from child-to-parent (p = 0.003) predicted new 10-month PA co-engagement. Discussion: Providing FHH information on a household level can promote parental encouragement for PA/HW, which can promote greater parent-child co-engagement in PA. In this high-risk population with a cultural emphasis on family ties, using FHH-based risk information for all adult household members may be a promising avenue to promote PA.