This study examines the factors that influence employee participation in workplace mental wellbeing initiatives. Extant research consistently shows that employee participation rates are low, typically between 1–50 percent. However, there is limited research explaining why this is the case, due, in part, to the underrepresentation of employees’ perspectives, an overreliance on quantitative approaches to identify barriers to participation, and a resulting lack of qualitative research that provides in-depth insight into the factors that both impede or enhance participation. To address these gaps in the literature, a qualitative case study was conducted at a large local government organisation in New Zealand. Data was collected from organisational records and semi-structured interviews with 19 employees, five managers, and the company’s Employee Assistance Programme (EAP) provider. The thematic analysis identified six factors that influence employee participation: 1) leadership, 2) organisational context, 3) perceived value, 4) remaining silent, 5) work pressures and expectations, and 6) initiative specific factors. This study proposes that three key leadership behaviours of obstructing, accommodating, and promoting have a significant and direct influence on employee participation, and also indirectly moderate how employees experience each of the other factors. The study demonstrates that employee participation is heavily influenced by how leaders shape the organisational culture and their ability to foster a work environment that is characterised by high levels of trust and psychological safety. This study makes a meaningful contribution to the literature on mental wellbeing by highlighting the dynamic interplay between a range of individual and organisational factors that can both positively or negatively influence participation. Importantly, this thesis provides practical recommendations for organisations seeking to enhance sustained employee participation in workplace mental wellbeing initiatives.