When addressing child and youth mental health, policy makers around the world call for collaboration between welfare providers. Research shows, however, that cross-sector collaboration is challenging. This article aims to scrutinize the issue of sustainability in the collaborative work undertaken between welfare providers to jointly support and promote child and youth mental health. In a qualitative interview study, 19 key officials involved in collaborative mental health work in three Swedish municipalities were interviewed, 13 individually and 6 in three small groups. Data were analyzed through content analysis and the application of practice-oriented collaboration theories. The results show that informants feel collaboration is beneficial for child and youth mental health. The results also show that five aspects of this collaborative work can affect its sustainability: (1) how the collaborative work was set up: if it was a special project or part of existing organizational structures; (2) what model of funding was used; (3) how many organizational levels were involved; (4) if goals were common, concurrent or contradictive; and (5) if important stakeholders were seen to be 'missing'. Collaboration members felt their collaborative work had caused them to drift away from important non-participant stakeholders. This article concludes that to develop long-term sustainable collaborations addressing child and youth mental health, key features of collaborative work need to be taken into consideration.