Affect dysregulation (AD) is characterized by heightened reactivity to strong emotions, difficulty calming down when upset, and mood instability. This phenomenon has not been widely examined in older foster youth, yet it may be an avenue to improve behavior and functioning in young adulthood. This study examines two dimensions of AD—affect skills deficits and affect instability—in a sample of 17-year-old foster youth, assessing the relationship of each dimension to risk factors and behavioral health service use at age 17, and as predictors of functional outcomes at age 19. We found that the level of AD among older foster youth was similar to a clinical sample and was associated with a history of physical abuse, depression, attention deficit hyperactivity disorder, and disruptive behavioral disorders. Higher levels of affect skills deficits were associated with use of intensive types of services such as psychiatric hospitalization, residential treatment, and psychiatric medications but affect instability was not. Higher levels of affect skills deficits were negatively related to graduating from high school and positively related to being arrested. AD, especially affect skills deficits, are a promising target for inclusion in interventions to support older foster youth with mental health problems.