HP in the coming days expects to acquire the remaining outstanding shares and close the acquisition.
Mercury develops testing and business technology optimisation software that allows IT departments to automate the management of systems.
The firm will be integrated into HP's existing software group which includes the HP OpenView management application.
Mercury became an acquisition target after it was caught in a scandal over the backdating of stock options which forced its chief executive to resign last year.
The deal was first announced in July and at the time was expected to close before the end of this year.
HP extended its offer four times as the company waited for Mercury to publish its restated earnings reports.
The acquisition marks the first multi-billion dollar purchase by HP since it acquired Compaq in 2001.
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