The transition from adolescence to adulthood is a highly vulnerable period for mental health concerns and is a key life-stage, fundamental to later health and quality of life. Scant research has investigated the health outcomes of young adults admitted to a specialised acute psychiatric unit for this cohort. Understanding health and functional outcomes for young adults admitted to a specialised acute mental health inpatient unit is beneficial in identifying treatment effectiveness and vulnerable groups. The study examines the relationship between demographic and clinical factors with admission, discharge and change in Health of the Nation Outcomes Scale (HoNOS) scores in patients admitted to an acute young adult psychiatric unit. The HoNOS was completed at admission and discharge for patients admitted to an acute young adult psychiatric unit in Australia. Descriptive statistics, paired t-tests, generalised estimating equations, independent samples t-tests and correlations were completed. Patients (N = 130) were aged between 18 and 25 years old (M = 21.26 years old, SD = 2.31). Patients were diagnosed with mood disorders (21.54%), substance abuse disorders (20.00%), personality disorders (19.23%), and psychotic disorders (17.69%). HoNOS scores across all subscales improved from admission to discharge, when controlling for confounding variables. Admission, discharge and change in HoNOS scores were associated with gender, country of birth, number of admissions, mental health act, diagnosis and socioeconomic status. This study is the first to explore HoNOS scores, demographic factors and clinical variables in young adults admitted to a specialised psychiatric inpatient unit for this age group. While further research is required, the findings identifies vulnerable subgroups (e.g., individuals with personality disorders, individuals with substance use, those admitted involuntarily, those readmitted) that may benefit from increased tailored support to foster more optimal health outcomes.