I examined whether neighborhood disadvantage directly associates with concurrent and longitudinal type 1 diabetes (T1D) outcomes among late adolescents transitioning into early emerging adulthood, and whether such disadvantage indirectly links to outcomes through parent–adolescent relationship quality. Seniors in high school with T1D (N = 236; mean age 17.76 ± 0.39 years; 61% female; 73.7% non-Latino White) completed self-report measures of relationship quality with mothers and with fathers, and adherence to their diabetes regimen; glycemic control was measured using HbA1c assay kits. Both T1D outcomes (i.e., adherence, HbA1c) were assessed annually across the following two years. Census tract indicators of neighborhood disadvantage (e.g., % unemployed) were culled from participant addresses at baseline linked to American Community Survey data. Structural Equation Modeling was used to estimate direct and indirect links between neighborhood disadvantage and both concurrent and longitudinal trajectories of diabetes outcomes. All models showed excellent fit to the data. Greater neighborhood disadvantage associated with lower relationship quality with both parents. In models examining mother and father relationship quality separately, lower relationship quality with mothers directly linked to poorer concurrent adherence, and formed an indirect path linking neighborhood disadvantage with poorer concurrent adherence. Lower relationship quality with fathers directly linked to poorer concurrent adherence and glycemic control, and formed an indirect path linking neighborhood disadvantage to both outcomes concurrently. When relationship quality with each parent was modeled simultaneously, the indirect path linking neighborhood disadvantage to concurrent adherence through relationship quality with fathers remained significant, while the path through relationship quality with mothers did not. Neighborhood disadvantage and parent-adolescent relationship quality were generally unrelated to longitudinal trajectories of diabetes outcomes. Parent-adolescent relationship quality (especially with fathers) remains important for T1D outcomes among late adolescents on the cusp of emerging adulthood, but may be at risk among those living within a socioeconomically-disadvantaged neighborhood.