In the current investigation, the processes of functional assessment and function-based intervention were used to resolve the severe challenging behaviors of a boy with autism and mental retardation in an inclusive kindergarten in South Korea. A multicomponent intervention was developed in collaboration with classroom personnel and was implemented entirely by the teacher and an aide in the context of a multiple-baseline-across-activities experimental design. Results were empirical validation of hypotheses derived from the functional assessment, as well as lower levels of challenging behaviors and increased rates of appropriate behaviors associated with the intervention. Positive interactions with a designated classroom peer and with the teacher also increased. The findings are discussed as contributions to the growing literature on functional assessment and function-based supports and the importance of promoting successful inclusive experiences for young children with disabilities.