Well before the green revolution in the 1960s, hybrid maize technology that had originally been developed in the USA spread across the world, starting before the Second World War. This article uses a framework that analyses the type of transfer (materials, knowledge, or capacity), the roles of diverse actors, and farmer demand and its market context, to trace the diffusion of hybrid technology to Latin America, Asia, Europe, and Africa up to 1970. The article also highlights the importance of access to diverse germplasm from the Americas provided by indigenous farmers. A handful of US public institutions promoted the spread of hybrid technology, with US private seed companies sometimes playing a secondary role. However, most cases of successful transfer were led by national scientists embedded in local institutions, who were able to link to local seed systems and farmers. By the mid 1970s, the aggregate impacts of these efforts were of the same magnitude as for the well-known and much publicized green revolution wheat varieties. Nonetheless, adoption of hybrid maize across and within countries was very patchy, relating to differences in scientific capacity, type of farmer, agro-ecology, and complementary investments in seed systems and extension. Consequently, impacts were often highly inequitable.