Mechanisms that large organizations employ to facilitate corporate social responsibility (CSR) engagement simply do not apply to start-ups due to distinct differences. The purpose of this study was to gain insight into how start-ups strive for sustainability in their business models by investigating internal and external drivers related to organizational processes, managerial characteristics, and stakeholder expectations. We explored key factors such as decision-making regarding CSR engagement, business values about sustainability, entrepreneurial orientation, and the relevance of the CSR theater (philanthropic orientation, disruptive innovation, or transforming the ecosystem). Multiple case studies and interview data elucidated how start-ups engage with their community and stakeholders to determine the best approach to sustainability demands, how start-ups embed sustainability practices within their business models, and how these practices match with the entrepreneurs’ personalities. On the basis of our case studies and data analysis, we propose that the decision to engage in CSR is treated as an investment decision. The business values of a start-up determine its CSR engagement. The philanthropic drive of a start-up determines its CSR initiatives, which are then in line with the field the start-up is operating in. Entrepreneurs’ willingness to adopt CSR practices is determined by their personalities and organizational expertise and experiences. CSR engagement within the business models of start-ups is based on a combination of financial and social capital, while financial benefits act as a continuous motivator for CSR engagement from inception.