The current study explored the effect of an early intervention (EI) service for psychosis in Hong Kong (EASY) on the reduction of the duration of untreated psychosis (DUP) at 10 years, in adolescent and adult patients. Data from three first-episode psychosis (FEP) studies conducted over 10 years using the same DUP assessment was analysed. FEP study 1 year before the EI service (2000) was used as a control, with those aged <25 comprising the youth psychosis control group and those aged over 25 comprising the adult psychosis control group. Data from the EASY service collected in 2010 constituted the youth psychosis EI group and data from an adult EI service collected from 2009 to 2011 constituted the adult psychosis EI group. DUP of EI and control groups was compared. The effect of the mode of onset and family history of psychosis on DUP was studied. DUP in the adult group was significantly reduced from median of 180 days to 93 days after 10 years (U = 12 906.50, P = 0.01), particularly in those with gradual onset and without family history. However, no significant change of DUP was found in the youth psychosis group. Results of the current study provide evidence that EI programmes with public awareness campaign can significantly reduce DUP of adult patients but not in youth group. The effect is more specific in those with long DUP. Further research should explore the specific factors contributed to the long DUP of patients of different age groups so as to establish targeted intervention strategies.