We explore one of the key underlying mechanisms that mediate the HRM–performance link, namely HRM implementation by line managers. The purpose of our study is to examine the role of various sources of support in explaining effective HRM implementation by line managers. Results indicate that HR, supervisor, and coworker support are relevant in explaining line managers’ HR role performance. In addition, younger line managers and those with greater supervisory experience are found to perform better in their HR role. Hence, organizations should consider developing a broad support network to ensure effective implementation of HRM by line managers.