He joins director of rugby Conor O'Shea, head coach John Kingston, skills coach Collin Osbourne and forwards coach Tony Diprose in agreeing a new deal; the quartet all signed extensions earlier this year.
Forty-year-old Mapletoft, capped by England in 1997, represented Quins during his playing career, and returned to the club in 2010.
The former England Under-20 coach said: "When Conor O'Shea offered me the opportunity to extend my stay with the club, it was a very easy decision.
"Harlequins is a fantastic environment to work in - supportive, challenging, fun and hugely rewarding to coach a large number of ambitious English players.
"Winning the Amlin Challenge Cup and the Aviva Premiership in successive seasons has given everyone involved with the club a boost, but we recognise that these successes are only a start."