An inkjet printing system includes a scalable printhead with an ink manifold. The scalable printhead is formed by mounting an ink manifold and multiple thermal inkjet printhead dies to a carrier substrate. The carrier substrate is machined to include through-slots. There is a through-slot for each refill slot among the multiple printhead dies. A first end of a given through-slot connects to a refill slot of a corresponding printhead die. An opposite, second end of the through-slot connects to the ink manifold. The ink manifold includes an inlet for coupling to an ink supply reservoir. The ink manifold also includes one or more channels and a plurality of feed openings. Each feed opening connects to a printhead die refill slot by way of a substrate through-slot.