Corporate social responsibility (CSR) and sustainable development practices and employer branding (EB) significantly influence the effectiveness of recruitment processes, human resource management (HRM), and an employer’s image. The main aim of this study was to assess the effectiveness of internal and external CSR practices in an international corporation operating in Poland. The empirical study consisted of two stages. In the first stage, an in-depth interview was conducted with a group of managers in the examined company. In the second stage, the company’s employees and local residents (non-employees) were surveyed with the use of questionnaires. The study demonstrated that CSR activities addressing environmental issues were most effective. External CSR practices received greater recognition among white-collar (office) employees, whereas blue-collar (manual) employees were more appreciative of internal CSR measures—in particular, financial support schemes. The effectiveness of internal and external CSR practices has been rarely compared in the literature, and the present study was undertaken to fill in this knowledge gap.