Background: Developing a good working alliance with clients with a severe mental illness (SMI) is a core competency of the employment specialist (ES). The ES's assessment of the working alliance was found to be related to the client's acquisition of a job in the regular market but we have little information on the processes and factors involved. Objective: To understand the development of the work alliance as assessed by the ES and its relationship to the client's acquisition of employment. Factors that may facilitate or hinder the development and evolution of the alliance were also explored. Methods: Cluster analysis was used to define alliance development patterns, while frequency analyses were used to identify differences between the patterns in terms of whether the clients with SMI obtained (or not) employment. Interviews with ESs explored factors that may have explained the different patterns. Results: Three patterns of working alliance were found and the one most often linked to client employment was the very high and stable pattern. The factors that might explain the different patterns are complex and interrelated. Conclusion: The results can be considered in the ES's initial and ongoing training on the working alliance and the implementation of quality supported employment programmes.