The merger of the world's top online search firm with the industry leader in matching ads to people's internet activities came after European regulators signed off on the deal, and strengthened Google's domination of the lucrative online ad business.
"We are thrilled that our acquisition of DoubleClick has closed,'' Google chairman and chief executive Eric Schmidt said in a written statement shortly after European antitrust regulators cleared the deal.
"Google now has the leading display ad platform.''
The European Commission said an investigation opened in November 2007 concluded the transaction "would be unlikely to have harmful effects on consumers''. US regulators approved the deal last year.
Google ended a bidding war with Microsoft in April 2007 by agreeing to pay $US3.1 billion ($3.4 billion) to add DoubleClick to its internet money-making arsenal.
"It's part of Google's desire to control most of the online ad revenue,'' said analyst Rob Enderle of Enderle Group in Silicon Valley.
"Google is trying to become a one-stop shop for ads.''
Google's purchase of DoubleClick is likely among the reasons Microsoft is now trying to buy Yahoo for $US44.6 billion in cash and stock, analysts said.