Tight-head props Collier and Sinckler will now remain with the current Aviva Premiership champions until the end of the 2014/15 season.
Head coach John Kingston announced that he was "absolutely delighted" about the news.
"Both have already had exposure in the first XV," he said. "I have no doubt that over the coming seasons both players will make a great impact in the Premiership.
"The development through the club's Academy is once again proof of Harlequins' considerable ability to nurture young English talent."
Collier, 21, learned about his rugby at Cranleigh School in Surrey before deciding to join Harlequins academy in 2008.
The Hammersmith-born forward represented England at Under-16 and Under-18 levels before becoming a member of the Under-20 team that reached the IRB Junior World Cup final in 2011.
Team-mate Sinckler has also represented England at junior levels, with the 19-year-old part of the Under-20s squad that won the Six Nations title in 2012.
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