Many youth with significant mental health (MH) and/or substance use (SU) difficulties do not receive specialized services. Collaboration between service providers, researchers, and other stakeholders is essential to improve youth service system capacity to provide evidence-based services to meet the complex array of needs of youth. Facilitators and barriers of implementing evidence-based practice have been identified, but few studies provide examples of the processes of collaboration and implementation for youth MH services. This study explicates the design features and implementation processes of a project to improve screening activities in youth services. These processes supported the building of 16 collaborative networks of service providers from diverse youth-serving sectors (e.g., MH, youth justice, child welfare) in urban, rural, suburban, and remote Canadian communities. These cross-sectoral networks implemented an evidence-based practice (screening youth aged 12–24 years for MH and SU problems using the Global Assessment of Individual Needs—Short Screener [GAIN-SS]) across their services. Materials and resources were provided by a centralized research team. Core project components were standardized and adherence to these components was monitored. Over 800 service providers participated in cross-sectoral networks, capacity-building events, joint data analysis, or interpretation and recommendation sessions. Across the 89 participating agencies, service providers for 84% of participating youth implemented the evidence-based practice accurately in accordance with project protocols, with 98% of positive screens reviewed and addressed according to organizational protocols. Service provider feedback is reported. Facilitators, barriers, and implications of promoting implementation of evidence-based practices across sites and sectors are discussed.