Brown, a product of the Premiership club's academy, has amassed 207 appearances and scored 67 tries for Quins since making his first team debut in 2005.
"If there's a person who embodies what it means to compete for his club it's Mike," Harlequins director of rugby Conor O'Shea said.
"He gives 100 per cent in everything he does and over the past couple of seasons he has received the recognition he deserves.
"The great thing about Mike is that, no matter what he achieves, he just becomes hungrier and hungrier for more and that is what you want from your players."
Brown has won 18 caps since making his Test debut in 2007 and is a member of England's senior elite player squad.
"There is so much potential at Harlequins to challenge in all competitions and I want to continue to be a part of that," the 27-year-old said.
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