Society is demanding more sustainable and socially responsible business models. Therefore, the concept of sustainability has become a cornerstone to help understand the success of many firms in the current competitive context. However, the context of SMEs has received little attention thus far. In order to solve this gap this article analyses the links between sustainability practices and business outcomes—both financial and non-financial (i.e., image and reputation)—for small and medium-size enterprises (SMEs). In addition, the study strives to analyze the potential differences between family firms and non-family firms. To this end, a quantitative study is carried out using PLS techniques to analyze a sample of SME owners and managers with a view to testing the proposed model based on the Stewardship Theory and Socioemotional Wealth Theory. In this sense, our study is pioneering in that it aims to assess—from a quantitative viewpoint—the moderator role of family firms on a series of relevant sustainability-driven outcomes. The data suggest that, in SME contexts, sustainability influences the corporate reputation, brand image, and financial value of the company. Importantly, we find that the profile (family vs. non-family) of the firm moderates the links between sustainability and business outcomes. Hence, our findings have important implications for sustainability implementation in SME contexts. Finally, we provide a series of guidelines aimed at maximizing the effectiveness of sustainability-based business practices.