Despite a growing literature associating physical discipline with later child aggression, spanking remains a typical experience for American children. The directionality of the associations between aggression and spanking and their continuity over time has received less attention. This study examined the transactional associations between spanking and externalizing behavior across the first decade of life, examining not only how spanking relates to externalizing behavior leading up to the important transition to adolescence, but whether higher levels of externalizing lead to more spanking over time as well. We use data from the Fragile families and child well-being (FFCW) study to examine maternal spanking and children's behavior at ages 1, 3, 5, and 9 (N = 1,874; 48% girls). The FFCW is a longitudinal birth cohort study of children born between 1998 and 2000 in 20 medium to large US cities. A little over a quarter of this sample was spanked at age 1, and about half at age 3, 5, and 9. Estimates from a cross-lagged path model provided evidence of developmental continuity in both spanking and externalizing behavior, but results also highlighted important reciprocal processes taking hold early, with spanking influencing later externalizing behavior, which, in turn, predicted subsequent spanking. These bidirectional effects held across race/ethnicity and child's gender. The findings highlight the lasting effects of early spanking, both in influencing early child's behavior, and in affecting subsequent child's externalizing and parental spanking in a reciprocal manner. These amplifying transactional processes underscore the importance of early intervention before patterns may cascade across domains in the transition to adolescence.