The England scrum-half has made 116 appearances since making his debut for the Aviva Premiership champions.
His career threatened to be derailed by a series of off-field incidents but he was a key member in Quins' march to league glory and he returned to the international set-up in June.
Care is one of several contenders to fill one of the scrum-half slots during the 2013 Lions tour to Australia and he will enjoy a stable build-up after re-signing.
"I'm enjoying being part of a talented squad that is playing an attractive and successful style of rugby," said Care.
"There is no other club that I want to play for so I'm pleased to extend my contract with Harlequins.
"I'm enjoying being part of a talented squad that is playing an attractive and successful style of rugby.
"Winning trophies is why we play professional rugby and having tasted some success in that regard, I would like more."
The most damaging of Care's turbulent spell off the pitch culminated in a £3,100 fine and driving ban after being arrested in the early hours of New Year's Day.
Care was booted out of England's RBS 6Nations squad as a result and slapped with a £10,000 fine from his club.
But Care has put his career back on track and demonstrated his talents during England's June tour of South Africa, scoring a try in the third Test.
Harlequins director of rugby Conor O'Shea was delighted to secure Care's long-term future and reckons he can continue to shine on the pitch.
"For Danny to commit to us for this length of time is news that everyone at the club is delighted about," added O'Shea.
"I'm looking forward to watching Danny's continuing development both on and off the pitch over coming seasons and to seeing him become the player that we all know he can be."