Answering the call for more insights in the relationship between human resource management (HRM), employee well‐being and performance, this study examines the link between developmental human resource (HR) practices and employee task performance, and includes both happiness (i.e., affective organizational commitment) and health related (i.e., exhaustion) well‐being as mediators. Based on social exchange theory and cognitive dissonance theory, we also explore the line manager's developing leadership behaviour as a moderator in the relationship between developmental HRM and well‐being. The multi‐source data from 403 employees and 53 line managers show that distinct developmental HR practices influence well‐being and employee performance differently, and suggest that the developing leadership behaviour of line managers influences the way in which developmental HR practices affect employees. Overall, the results underline the need for a configurational perspective on HRM, well‐being and employee performance, taking line managers’ behaviour into account as an essential element of the HRM system, next to formal HR practices.