Abid Naseer, 26, flew out of Britain from Luton Airport near London under US custody and was en route to New York. He is due to face a federal judge on Monday on charges of joining a failed Al-Qaeda plot to bomb the Big Apple.
Naseer, from Manchester in northern England, "was extradited to America where he is accused of terrorism offences. His case is now a matter for the US authorities," Britain's Home Office said.
Naseer was originally arrested in Britain along with 10 other Pakistani men in 2009 over a suspected Manchester bomb plot. They were released without charge after prosecutors cited inadequate evidence, and ordered to be deported.
An immigration judge subsequently ruled that despite Naseer being "an Al-Qaeda operative who posed and still poses a serious threat," he could not be returned to Pakistan as his safety there could not be guaranteed.
Two months after his release, in July 2010, Naseer was arrested again on a US warrant linked to the New York case.
According to American prosecutors, Naseer was a go-between for three men convicted of traveling to Pakistan for militant training, then plotting in New York in 2009 to set off suicide bombs in the city's subway.
The plotters admitted to communicating with an Al-Qaeda organizer in Pakistan named Ahmad. US authorities say Ahmad was in turn communicating with Naseer, who likewise visited Peshawar, Pakistan.
"After returning to the United Kingdom, Naseer sent messages back and forth to the same email account that 'Ahmad' was using to communicate with the American-based Al-Qaeda cell," federal prosecutors said in a statement.
In one email, Naseer allegedly used coded language that a "wedding" was ready, in reference to a bombing, echoing a message from one of the New York men who wrote "the marriage is ready" before he tried to complete the plot.
In January last year, a judge approved Naseer's extradition to the United States.
He appealed to the European Court of Human Rights but his case was thrown out in December.
Metropolitan Police officers on Thursday escorted Naseer from Belmarsh Prison to Luton Airport where he was handed over to US authorities.
"The defendant is one of a long line of terrorist suspects extradited to these shores and this courthouse to face justice for their efforts to wreak havoc here and overseas," US Attorney Loretta Lynch said in a statement.
"As alleged, this defendant was instrumental in one tentacle of an international plot that reached to New York, Norway, and the United Kingdom."