As massive open online courses (MOOCs) proliferate, a better understanding of MOOC instructors is essential. This study examined the motivation as well as the career and professional development of 142 MOOC instructors using a mixed-methods approach entailing an online survey combined with six MOOC instructor interviews. The research findings indicated that instructors' motivation to teach MOOCs primarily related to intrinsic motivation. Importantly, this study classified the motivational factors of MOOC instructors into seven categories. While the frustrations of MOOC instructors included time for creating MOOCs and a lack of interaction in MOOCs caused, in part, by heavy reliance on asynchronous communication, many perceived that MOOC teaching positively influenced their professional development. Most MOOC instructors learned how to teach MOOCs informally and individually. Nevertheless, they tried to help other MOOC instructors. This study points to the strong need for professional development of MOOC instructors in developing and effectively MOOCs.