Objectives <p>Recent research suggests mindfulness as a promising approach to support children’s social-emotional well-being, but the meaning of mindfulness interventions to children’s well-being is rarely explored from the children’s perspective. Using Chinese migrant children as an example, this study explored vulnerable children’s experiences of a school-based mindfulness intervention.</p> Method <p>This study was part of a large-scale cluster randomized controlled trial and adopted a mixed-methods design, combining qualitative and quantitative analytic strategies. Twenty-nine migrant children (mean age = 10.3&#xa0;years) participated in in-depth interviews after a 12-week intervention. Children’s self-portraits representing their perception of self before and after the intervention were collected during the interviews. Interview transcripts were analyzed using reflexive thematic analysis, and an exploratory quantitative analysis examined differences in drawing features between these two self-portraits with visual elements potentially linked to mental well-being.</p> Results <p>Thematic analysis elicited two key themes and five subthemes: (1) emotional well-being, which included two subthemes, body–emotion awareness and emotion regulation; and (2) social well-being, which included three subthemes, impulse control, prosocial behaviors, and kindness. Drawing analysis complemented the narrative findings, illustrating how children used drawings to express psychosocial change. A preliminary Mindful–Social–Emotional Model is proposed to organize children’s descriptions of their perceived benefits.</p> Conclusions <p>This study highlights the potential value of school-based mindfulness programs in promoting social-emotional competencies among vulnerable children. Drawing-based interviews provided a developmentally appropriate means to access children’s voices and perspectives on change.</p> Preregistration <p>This study is not preregistered.</p>