This article examines the relationships between family socioeconomic status (SES) and mathematics achievement across countries while taking into account (a) the country's development status and (b) the types of resources (physical versus human) available at home. The 2012 round of the Programme for International Student Assessment (PISA) data were analyzed using the necessary condition analysis (NCA) procedures. The study found that only the human resource aspects of family SES, that is, parental education and parental occupational status, were statistically significant necessary conditions for mathematics achievement across more developed, OECD countries. By contrast, none of the 10 SES variables showed statistically significant, necessary conditions for mathematics achievement across less developed, non-OECD countries. Therefore, the necessary conditions for countries' mathematics achievement differ depending on their developmental status and by different measures of family SES. Some practical implications are discussed along with limitations and directions for future studies.