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.
Having helped O'Shea's side to Premiership glory against Leicester at Twickenham in May, he is delighted to have extended his stay at the Stoop.
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."