Jassar, a 25-year-old engineer, had pleaded not guilty at the start of the one-day trial and said that his confession had been extracted through torture.
"I had nothing to do with this and I am innocent," he said, before being sentenced.
Jasser was the second person to be found guilty over the killing.
Hassan had been head of the Care International humanitarian group in Iraq for 12 years when she was pulled from her car in Baghdad's Jamaa district.
Several video messages were released by Hassan's kidnappers, showing her appealing for her life and calling for the withdrawal of British troops from Iraq.
Hassan held British, Irish and Iraqi citizenship and was married to an Iraqi.
Hassan's family said in 2006 that while she was being held, her husband Tahseen had received four telephone calls from her kidnappers, which were made using her mobile phone.
They said that the hostage-takers, who called themselves "an armed Islamic group", had demanded to speak to a member of the British embassy, but it had refused to have any contact with the kidnappers.
The group had initially threatened to hand Hassan over to Abu Musab al-Zarqawi, the now-dead leader of al-Qaeda in Iraq, unless British troops left the country.
The kidnappers also managed to directly get in touch with embassy officials after the killing, demanding $1 million for the reutrn of her body.
Deirdre Fitzsimons, Hassan's sister, expressed hope that Jassar would now reveal the location of Hassan's remains to allow for a proper burial in Britain.
"My sister was a Catholic and it would be her wish to have a proper Christian burial," she said.
"However much she loved Iraq, she always wanted to be buried in this country. That is what we want to do for her."