Lindhout and Brennan were captured on August 23, 2008 as they travelled from Mogadishu to a refugee camp in Afgooye.
A Somali journalist and two drivers were also taken hostage but freed after 177 days. They were unable to identify their captors or the motives for the kidnappings.
In September 2008, a local tribal chief participated in negotiations to try to free the pair and said the kidnappers wanted $2.5m. In January, he said the demand had been reduced to $100,000.
On Wednesday night, Lindhout and Brennan were staying on a hotel in Mogadishu before being flown out of the country.
Kept in chains
Brennan said that when he was bundled into a car before being released he feared that he was being handed over to one of the Islamist groups fighting government forces in Somalia.
"Tonight we were ripped out of our rooms, stripped of everything, told to put on new clothes and then thrown in a car and then driven - we had no idea what was going on," he said by telephone from the hotel.
"We were always kept by the same people. That was especially one of my concerns towards the end. They were talking about selling us to another group."
Brennan said he was kept in chains and had been isolated from Lindhout for 10 months after they had tried to escape.
"Locked in a room, very little light. I haven't been able to exercise at all. You know, simple things like being told not to smile, not to laugh - not that there was much to laugh about," he said.
"But my friend Amanda had a pretty rough time, I know that. She was severely beaten and we were both tortured both mentally and physically.
"Being pistol-whipped is sort of torture, being completely stripped of everything and then locked in a room, no one to speak to, is a form of torture really."
There are frequent abductions of journalists and aid workers for ransom in Somalia, but the 15 months that Lindhout and Brennan spent in captivity was unusually long.
Large areas of Somalia are controlled by anti-government groups, which are battling an administration that has little effective control over much of the country.