Purpose: We aimed to examine whether wellbeing, health behavior, and youth life among young people (YP) with co-occurrence of physical-mental conditions, that is, multimorbidity differ from YP with exclusively physical or mental conditions. Methods: The population included 3,671 YP reported as having a physical or/and mental condition from a Danish nationwide school-based survey (aged 14–26 years). Wellbeing was measured by the five-item World Health Organization Well-Being Index and life satisfaction by the Cantril Ladder. YP's health behavior and youth life were evaluated in seven domains: home, education, activities/friends, drugs, sleep, sexuality, and self-harm/suicidal thoughts, in accordance with the Home, Education and employment, Eating, Activities, Drugs, Sexuality, Suicide and depression, and Safety acronym. We performed descriptive statistics and multilevel logistic regression analysis. Results: A total of 52% of YP with physical-mental multimorbidity reported a low level of wellbeing, compared to 27% of YP with physical conditions and 44% with mental conditions. YP with multimorbidity had significantly higher odds of reporting poor life satisfaction, compared to YP with exclusively physical or mental conditions. YP with multimorbidity had significantly higher odds for psychosocial challenges and health risk behavior, compared to YP with physical conditions, along with increased odds for loneliness (23.3%), self-harm (63.1%), and suicidal thoughts (54.2%), compared to YP with mental conditions. Discussion: YP with physical-mental multimorbidity had higher odds for challenges and low wellbeing and life satisfaction. This is an especially vulnerable group and systematic screening for multimorbidity and psychosocial wellbeing is needed in all healthcare settings.