Shaker Aamer, a Saudi national married to a Briton, was suspected by US authorities of being an Islamist militant associated with al Qaeda but was never charged with any crime.
Television images from Biggin Hill airport south of London showed the arrival of a small white aircraft. A government spokesman said Aamer was now back in Britain.
Britain said last month that Aamer would be released, and Foreign Secretary Philip Hammond said earlier on Friday that the former detainee was en route to Britain, a move welcomed by supporters.
"Today is a day for welcoming him back and hoping that he is healthy and well and that can join his family at long last," Kate Allen, UK Director of Amnesty International, told Sky News.
In Washington, the Pentagon said it reviewed Aamer's case and concluded he should be released.
"As a result of that review, which examined a number of factors, including security issues, Aamer was unanimously approved for transfer by the six departments and agencies comprising the task force," a Pentagon statement said.
Aamer was cleared for release by U.S. authorities in 2007 but not freed until now. During a debate in parliament earlier this year, some British lawmakers expressed concern that Aamer was still in Guantanamo because he "had seen too much" and could be still pose a threat to America if released.
Rights group Reprieve said Aamer moved to Britain in 1996 and was in Afghanistan in 2001 doing voluntary work for an Islamic charity when he was captured by Afghan Northern Alliance forces and handed to the U.S. military.
He was then transferred to Guantanamo when the prison camp was opened in 2002.
Several British lawmakers long argued for Aamer's release with John McDonnell, now the opposition Labour party's finance spokesman, telling parliament earlier this year that Aamer had "endured harsh, and brutal and inhuman treatment".
Allen said Britain should hold a "proper independent inquiry" into allegations including mistreatment.
Prime Minister David Cameron's spokeswoman said the government had no plans to detain Aamer but would do "anything necessary to ensure public safety".
Most Guantanamo detainees have been held without trial for over a decade and Washington has drawn international condemnation for harsh treatment of foreign terrorism suspects.
Labour leader Jeremy Corbyn said it was now time to close the facility, a vow President Obama made when first elected in 2008 but has not upheld. Congress has blocked any such move with provisions such as bans on transferring detainees to U.S. soil.
"Now Shaker has been released, the scandal of the Guantanamo detention camp itself must be brought to an end," Corbyn said.
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