Multi-purpose clinically relevant measures such as the Child and Adolescent Needs and Strengths (CANS; Lyons, 2009) and the Global Appraisal of Individual Needs (GAIN; Dennis et al., 2003) can be useful for improving services at the individual client, program, organization, and system levels. Yet, emerging research suggests that such measures are often not used consistently or effectively (Mellor-Clark, Cross, Macdonald, & Skjulsvik, 2016), and that poor use of these measures can be in part attributed to how they were put into practice (de Jong, 2016). Systematically conducted, empirical research on the effective implementation of such tools is scarce (Boswell, Kraus, Miller, & Lambert, 2015). Thus, the current study examined the factors and processes that contribute to the effective implementation of clinically relevant measures, specifically the CANS and GAIN, in community-based mental health agencies serving children and adolescents. A second objective was to examine the role of staff participation in the implementation process. Three general research questions guided the study, including: (1) How can clinically relevant measures such as the CANS be implemented effectively? (2) What are the perceived consequences of staff participation in adapting and implementing a version of the CANS and how do these consequences come about? and (3) How does the implementation context affect the process and its outcomes? The study employed qualitative, multiple-case study methods. Four child and youth mental health agencies in Ontario participated, including a total of 44 staff with varying roles (e.g., frontline and management). Several cross-case and within case comparisons were made to examine the contribution of staff participation and tool features, such as tool adaptability, to implementation outcomes. Data was analyzed using guidelines developed by Yin (2009), Miles and Huberman (1994), and Thomas (2006). Results suggest that staff participation in the process of putting clinically relevant measures into practice contributes to effective implementation and increased uptake and use of the measures. When staff are engaged in the process, they have reasons and opportunities to interact, talk about the use of the measure, and “co-create fit” between the measure and their work context. This improved fit then facilitates increased staff commitment and ability to use the measure effectively. Agency leaders play a key role in enabling this fit-making process through: encouraging and supporting a participatory approach to implementation, creating implementation structures, following through with planned activities, and being open and responsive to staff feedback. Findings suggest that the implementation context provides incentives or reasons for implementing a measure, affects the initial fit between the measure and staff members’ work, and affects the feasibility of engaging staff in the fit making process. In conclusion, this study is one of the few empirical studies to examine implementation of clinically relevant measures. The findings have important implications for research and practice, which will be discussed.