Research has shown that a variety of organisational change interventions can be effective but the powerful positive results of an intervention do not always generalise to other similar settings. Problems with implementation and a difficult intervention context have been shown to undermine the effectiveness of promising interventions. The impact that middle managers have on the change process and intervention outcomes has not been widely researched. This longitudinal intervention study was carried out in the elderly care sector in a large Danish local government organisation (N = 188), where poor social support, and lack of role clarity and meaningful work had been identified as significant problems. To tackle these problems, teamwork was implemented, with teams having some degree of self-management. It examined whether middle managers' active support for the intervention mediated its impact on working conditions, well-being and job satisfaction. Structural equation modelling showed that middle managers' active involvement in implementing the change partially mediated the relationship between working conditions at time 1 and time 2. Working conditions at time 2 were in turn related to time 2 job satisfaction and well-being. These results suggest that the degree to which employees perceive their middle managers to play an active role in implementing change is related to intervention outcomes.