Unveiling Hart's new role as his personal representative to the talks, US Secretary of State John Kerry called him "one of our country's most respected and accomplished senior statesmen, creative and strategic thinkers, and my longtime friend."
Kerry said he had asked Hart to "support the parties in Northern Ireland as they enter a new round of talks to achieve a lasting peace."
"Gary is known as a problem-solver, a brilliant analyst, and someone capable of thinking at once tactically, strategically, and practically," Kerry said in a statement.
Talks to end the impasse in Northern Ireland began in Belfast last week, with British Prime Minister David Cameron urging all sides to work "constructively."
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Northern Ireland's assembly has been deadlocked for months over budget disputes, as well as rows over divisive sectarian issues such as displays of flags and parades.
The signing of the historic Good Friday Agreement peace deal in 1998 largely brought an end to 30 years of unrest known as "The Troubles," although violence periodically flares.
But outstanding issues remain and four months of lengthy negotiations chaired by former US diplomat Richard Haass last year ended with no deal.
Kerry said Hart had been working as chairman of the State Department's International Security Advisory Board and chairman of the Threat Reduction Advisory Council at the Department of Defense.
"Now we're fortunate that he's agreed to devote some additional time to engage in the tough and patient work of diplomacy as my personal representative, including on issues related to Northern Ireland. He does so with my confidence and trust," Kerry said.