Footage of Mr Curtis was released in June showing him dishevelled and with a long beard. He reportedly said he had everything he needed and “everything has been perfect – food, clothing, even friends now,” words which Al Jazeera suggested may have been read from a prepared script.
The statement was stark contrast to with the beatings and torture described by an American photojournalist who was held alongside him for seven months, The New York Times reported.
Matthew Schrier said he had been tortured and starved by masked jailers who, he said, had forced a car tyre over his knees, confined him with a wooden rod slid behind his legs and beat his feet so that he could not walk.
Mr Schrier said that he managed to escape last year by releasing wire caging and slipping through a hole in the wall.
He told the newspaper that Mr Curtis was heavier-set and became stuck when he tried to follow, eventually urging Mr Schrier to flee without him.
At the request of Mr Curtis’s family, media organisations did not identify him in their reports on Mr Schrier.
In a statement, Mr Schrier welcomed the release of his former fellow hostage, saying: "This day will go down as one of the happiest of my life.
"After living through the tragic events of this past week with the death of James Foley and hearing that the life of Theo Curtis has been spared, it gives me great hope for the safe return of all the other hostages."
Susan Rice, the US National Security Adviser, added that Mr Curtis should be back with his loved ones shortly.
She went on: "We will continue to work tirelessly on behalf of all Americans who are held overseas so that they can be reunited with their families as well."
Mr Curtis’s release came the same day as a German newspaper reported that a 27-year-old man from Brandenburg had been freed in June after negotiations with his captors. Security sources reportedly confirmed his Islamist kidnappers had received something for his release – but they did not specify what.
The death of Mr Foley, set against the release of the German man and Mr Curtis, has again raised the question of whether ransoms should ever be paid.
Both the families of the German and Mr Foley received ransom demands. Diane and John Foley, the American journalist’s parents, were working to raise the £80 million sum.
The American and British governments will not negotiate with or pay ransoms to kidnappers, believing any such transactions would only encourage further abductions. The payment of ransoms is illegal in the US.
The Gulf state of Qatar issued a statement saying it had brokered the release of Mr Curtis, who was not held by Isil, which has been disowned by al-Qaeda.
Qatar's foreign ministry said it: "exerted relentless efforts to release the American journalist out of Qatar's belief in the principles of humanity and out of concern for the lives of individuals and their right to freedom and dignity."
Mr Curtis's mother said the family was "repeatedly told by representatives of the Qatari government that they were mediating for Theo's release on a humanitarian basis without the payment of money."
A New York Times investigation last month found that al-Qaeda and its affiliates had raised at least $125 million (£75 million) from ransoms, including $66 million last year. The payments were mostly from European governments. Four French and two Spanish journalists held by Isil with Mr Foley were freed this year after ransoms were paid by their governments through intermediaries.
Mr Foley's family, meanwhile, offered prayers for the safety of his fellow hostages in Syria during a memorial mass his home town of Rochester, New Hampshire.