Objective: The aim of this study was to assess the impact of self-reported emotional and conduct problems on family functioning and quality of life (QoL) among adolescents with ADHD. Method: The ADHD group (N = 194) was divided into the following groups: without additional emotional or conduct problems, with emotional problems, with conduct problems, and with both problem types. The cross-sectional study included parent and adolescent reports. Results: Adolescents with ADHD and both problem types reported significantly lower QoL and family functioning than all other ADHD groups. Parents reported better QoL for the ADHD group without additional problems, but similar family functioning for all groups. Conclusion: A higher level of coexisting psychiatric problems had a significant impact on adolescents’ reports of family functioning and QoL. However, coexisting problems showed no association with parent reports of family functioning. Adolescents with ADHD might add important information in clinical assessment.