Mobile payments are reshaping the global payment landscape with some developing economies leapfrogging advanced economies in adoption. We build and estimate a dynamic model of sequential payment innovations-progressing from cash to card to mobile-to explain this pattern. The model matches cross-country payment technology adoption patterns and shows how advanced economies' early success in adopting card payments dampens subsequent mobile payment adoption. Extending the framework to a two-sided market with price coherence, we show that payment externalities justify policy intervention: Promoting mobile payment adoption and usage (e.g., via subsidies or price differentiation) enhances welfare, especially in developing economies.