Mustafa Salman was convicted of aiding and abetting the kidnappers, the official said on Monday.
The two other defendants in the case were freed.
Hassan, an Iraqi-British national who had lived in Iraq for more than three decades after marrying an Iraqi engineer, was head of the Iraqi operation of the charity Care International.
She was abducted while travelling to work in Baghdad in October 2004, and was killed about a month later after appealing in video messages made by her abductors for British forces to withdraw from Iraq.
No group claimed responsibility for the abduction or the killing, and her body has not been found.
The three men put on trial in Baghdad were captured by US forces in May last year in possession of Hassan's personal belongings, including her handbag and make-up.
Police said at the time that 11 people had been detained, and that five had admitted complicity in the killing.
Hassan, 59, was widely known in the aid community as a tireless worker for impoverished and marginalised Iraqis.
Sister blames Britain
On Monday, her sister accused the British government of condemning her to death by refusing to talk to her captors.
Deirdre Fitzsimmons told BBC radio that Britain had treated four phone calls from her captors as hoaxes.
"We were advised ... by the powers that be that these were hoax calls. But after all they were made on my sister's mobile telephone," she said.
Hassan's husband Tahsine was
left on his own, her sister says
"They were definitely coming from these kidnappers because they would make calls and then they would issue videos and they never made any demands for money.
"My brother-in-law [Tahsine Ali Hassan] was left in a house on his own without any recording equipment. He was given the advice to say the British didn't want to be involved.
"The last phone calls were made on November 7. They demanded to negotiate. I don't think he knew what to do. He did the best he could."
Hassan was murdered the following day.
Fitzsimmons appealed to the authorities to find out where her sister's body was so it could be returned to England for burial.
"These men know where my sister is buried and all we want to do now is to bring her home," she said.
Armed men killed at least 11 college students after stopping a bus in Baghdad and kidnapped up to 50 transport company employees in the Iraqi capital on Monday.
Interior ministry sources said the bus was carrying 20 college students in Baghdad's Dora district.
The sources said the seizure of the transport company employees occurred in broad daylight and that the kidnappers were dressed in police uniforms.
Italian convoys have been hit
before by roadside bombs
Police said the abductions appeared to be a co-ordinated operation against firms offering transport to Syria and Jordan.
Hours after the transport company employees were seized, assailants in police uniforms raided houses in Baghdad's Shaab district and kidnapped seven people, police sources said.
An Italian soldier was killed when a bomb blew up a vehicle in southern Iraq late on Monday.
The Italian army said four others were wounded in the attack near Nassiriyah in the south of Iraq.
A defence ministry official said the Italian soldiers' vehicle was hit by a roadside bomb and that one of the wounded suffered serious injuries.
The soldiers were escorting a British convoy on its way to Tallil, and were some 100km north of Nassiriyah about 1730 GMT when the attack occurred.