Over recent decades, there has been an important stream of research that has examined the human resources management (HRM) and organizational effectiveness relationship. However, few of these studies have paid attention to developing perspectives regarding effective HRM implementation. Such implementation is a process that is not only executed by HR managers, for it is also the responsibility of line managers and other employees of the organization. Increasingly, the responsibilities of HRM are being passed on to line managers, who have thus become more involved in dealing with their employees’ HRM issues. This study is aimed at investigating the factors that can influence the degree to which effective HR practices are implemented, going further than previous theoretical studies by proposing a conceptual model of HR implementation effectiveness. We incorporate the social context theory and AMO theory into the model. Social context factors of organizational culture, organizational climate, and political considerations as features of the work environment and top management’s HR support as a higher-order social context dimension are proposed as drivers for effective implementation of HR systems and practices. The model also considers ability-motivation-opportunity factors as one set of variables that mediates the relationship between social context variables and line managers’ HR implementation effectiveness. This study tries to step into an area of study which has not been comprehensively explored by the scholars. It involves putting forward a model of HR implementation effectiveness by considering the role of line managers and a set of propositions for testing in field. The propositions developed in the model provide guidelines for further study in the focal HRM topic that we believe will generate beneficial findings for both practitioners and academics.