A wide variety of peer worker roles is being introduced into mental health services internationally. Empirical insight into whether conditions supporting role introduction are common across organisational contexts is lacking. A qualitative, comparative case study compared the introduction of peer workers employed in the statutory sector, voluntary sector and in organisational partnerships. We found good practice across contexts in structural issues including recruitment and training, but differences in expectations of the peer worker role in different organisational cultures. Issues of professionalism and practice boundaries were important everywhere but could be understood very differently, sometimes eroding the distinctiveness of the role.