In business-to-business (B2B) markets, the success of key account management (KAM) teams depends on how they are structured and how they handle customer relationships. The authors conceptualize relationships among selling team members as a within-seller (intrafirm) network and the relationships between selling team members and buyer representatives as a buyer–seller (interfirm) network. Drawing on both structural (buyer–seller density, within–seller density, and within-seller centralization) and functional (buyer–seller similar function ties and within–seller cross-functional ties) composition attributes of these networks, the authors examine how the interplay between these networks drives seller account profitability. Using data from 207 key account managers across B2B industries, the authors uncover a nuanced pattern of interplay across the two networks. Densities in the two networks are mutually substitutive, but density in the buyer–seller networks and centralization in the within-seller networks serve complementary roles. Cross-function ties in the within-seller network serve a complementary role too, but only in relation to similar function ties in the buyer–seller network. In contrast, within-seller centralization supports both network density and similar function ties in the buyer–seller network and, thus, emerges as a valuable KAM team characteristic. These findings suggest multiple ways for firms to align interfirm and intrafirm KAM networks to improve account profitability.