High involvement work systems (HIWS) have been found to be improve employee well-being. The underlying processes through which HIWS influence employee well-being and the conditions under which these practices work are not fully understood. This study draws on job demands-resources theory to address this gap by theorising two novel mediators, that is, work pressure and bonding social capital, to explain how HIWS influence emotional exhaustion. We further proposed that engaging leadership as a proxy of line manager implementation of HIWS would strengthen these relationships. An integrated model is presented on how, why, and when HIWS influence employee well-being. Using data collected from 97 employees in a pharmaceutical company via a general survey and then a diary survey for 5 working days, this study found that HIWS alleviated day-level emotional exhaustion through their experience of higher day-level bonding social capital and lower day-level work pressure and these relationships were stronger under high level of engaging leadership.