Objective: To examine whether individual differences and day-to-day fluctuations in diabetes goal planning are associated with Type 1 diabetes (T1D) management during late adolescence, and whether lapses in daily diabetes goal planning are more disruptive to diabetes management among those with poorer executive functioning (EF). Method: Late adolescents with T1D (N = 236, Mage = 17.77 years) completed survey measures assessing individual differences in levels of diabetes goal planning and adherence, as well as survey and performance-based measures of EF; glycemic control was assessed through glycated hemoglobin (HbA1c) assays. Participants then completed a 2-week daily diary, rating items measuring daily diabetes goal planning, goal effort, and adherence, and recording blood-glucose tests from their glucometer at the end of each day. Results: Analyses of survey measures indicated that higher individual differences in diabetes goal planning were associated with better adherence and glycemic control. Analyses of daily data using hierarchical linear modeling indicated that adolescents displayed higher daily adherence and lower blood-glucose levels on days when they had higher-than-their-average levels of daily goal planning and daily goal effort. EF moderated the association between daily goal planning and daily adherence, indicating that lapses in daily goal planning were more disruptive for adolescents with poorer EF. Conclusion: Both individual differences and day-to-day fluctuations in diabetes goal planning are associated with diabetes management, highlighting the challenges of managing T1D in daily life. Youth in late adolescence with poorer EF may especially benefit from planning to attain diabetes goals on a daily basis.