Onitoni Large moved quickly through Manly's ranks, joining its Top 30 at 18 before making his NSW Cup debut in Round 11 and NRL debut in Round 26.
The playmaker, who turned 19 shortly before his NRL appearance, came off the bench and played in the halves.
Manly has now secured Onitoni and his younger brother Ashton until at least the end of the 2029 season.
Both featured prominently in the club's SG Ball Cup campaign and were selected for the NSW Under-19s State of Origin team this year.
Onitoni received Manly's Best Back of the Year award for its under-19s.
Ashton, a hooker previously in the Wests Tigers system, won the 2026 Jake Trbojevic Award for Junior Male Player of the Year and was Manly's SG Ball Cup Players' Player.
The retentions strengthen Manly's plans for the brothers, whom the club views as important prospects for its future spine.