The 24-year-old joined the Exiles at the start of the 2010/11 campaign and has since played 33 times for them.
Allinson has featured in nine of Irish's 15 games this season, forming an effective partnership with fly-half Dan Bowden.
"I have really enjoyed my first 16 months at London Irish and the opportunity to stay here for another two seasons was something I could not turn down," he told the club website.
"I have been playing my best rugby and with the help of the coaching staff have pushed my game onto the next level."
Exiles head coach Toby Booth was quick to praise Allinson's contribution to the team.
"I have been very impressed with Darren's work ethic since he joined the club. He has worked hard from day one and has earned the right to be a regular in the starting fifteen," said Booth.
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