Single case design research on family centered positive behavior support (PBS) over the past 20 years has provided evidence of the approaches acceptability, effectiveness and durability when implemented with families of children with developmental disabilities and problem behavior. Although quality of life is a key tenet of PBS, only a few studies of PBS with families have measured quality of life outcomes. The purpose of this study is to present family functioning results from the second half of a longitudinal study that investigated the consequential validity of an ecological approach to family centered PBS. The approach aimed to transform coercive into constructive family processes in family routines during a process of comprehensive assessment, multicomponent plan design and implementation support in collaboration with families. Ten families of children with developmental disabilities participated. Settings were 32 family routines, with two to four routines per family. Family functioning measures were family quality of life, parental stress, parental locus of control and social support. A repeated measures, quasi-experimental group design across mothers and fathers evaluated the statistical significance of changes in family functioning when comparing baseline to intervention and follow-up phases. Results evidenced significant improvements for mothers in family quality of life and parental stress during intervention and follow-up, and in parental locus of control during follow-up. Results evidenced significant improvements for fathers in parental stress during intervention and follow-up, but no significant improvements in family quality of life or parental locus of control. Results are discussed in terms of contributions to the literature, implications for practice, limitations, and future research.