Apathy is prevalent in first-episode psychosis (FEP) and associated with reduced global functioning. Investigations of the trajectory of apathy and its early predictors are needed to develop new treatment interventions. We here measured the levels of apathy over the first 10 years of treatment in FEP and in healthy controls (HC). We recruited 198 HC and 198 FEP participants. We measured apathy with the Apathy Evaluation Scale, self-report version, psychotic symptoms with the Positive and Negative Syndrome Scale, depression with the Calgary Depression Scale for Schizophrenia, functioning with the Global Assessment of Functioning Scale, and also estimated the duration of untreated psychosis (DUP). The longitudinal development of apathy and its predictors were explored using linear mixed models analyses. Associations to functioning at 10 years were investigated using multiple hierarchical linear regression analyses. In HC, mean apathy levels were low and stable. In FEP, apathy levels decreased significantly during the first year of treatment, followed by long-term stability. High individual levels of apathy at baseline were associated with higher apathy levels during the follow-up. Long DUP and high baseline levels of depression predicted higher apathy levels at follow-ups. The effect of DUP was persistent, while the effect of baseline depression decreased over time. At 10 years, apathy was statistically significantly associated with reduced functioning. The early phase of the disorder may be critical to the development of apathy in FEP.