US military jurors have cleared Osama bin Laden's driver of conspiracy charges but found him guilty of helping al-Qaeda.
The split decision has failed to end the controversy over the fairness of the US "war on terror" tribunals.
In the first full trial by the special tribunals in Guantanamo, the jury concluded Salim Hamdan, a Yemeni national, provided "material support" to the terror network by driving bin Laden and ferrying weapons.
Critics of the special tribunals argued the trial offered proof that the system set up to try terror suspects was fatally flawed, while US President George W Bush's administration said the result showed the process had been fair.
The White House praised the verdict after it was announced and the Defence Department said it intended to press forward with the trials of at least 20 more detainees held at the prison in Guantanamo.
"We're pleased that Salim Hamdan received a fair trial," White House spokesman Tony Fratto said.
"The Military Commission system is a fair and appropriate legal process."
"We respect that decision," Bryan Whitman, a Pentagon spokesman, said of the verdict.
"We fully intend to move forward with additional prosecutions on the 20 other cases that are currently in the military commission system."
The jury must now set a sentence for Hamdan, who faces a possible maximum term of life in prison.
At sentencing hearings that began on Thursday afternoon, the Navy officer presiding over the case, Keith Allred, said Hamdan would receive more than five years' credit for time served behind bars at Guantanamo since he was initially charged.
The judge also rejected a request by the prosecution to call an FBI agent to describe the effects of the attacks of September 11, 2001.
Allred said Hamdan was "such a small player" that it would be unfair to have the jury hear the testimony as it would imply Hamdan had a role in the attacks.
The case, the first US war crimes trial since World War II, was seen as an important test of the controversial military commission system that has been widely criticised as unfair by human rights groups.
Clad in a white turban and tan coat, Hamdan stood with a solemn expression and then bowed his head as the verdict was read out by the head of the jury in a makeshift courtroom at the remote US naval base in Guantanamo Bay, Cuba.
Hamdan was found guilty under the single charge of support for terrorism, alleging he served as a driver and bodyguard for bin Laden, ferried weapons for al-Qaeda and was fully aware that the organisation engaged in terrorism.
The rejection of the conspiracy charge showed prosecutors were unable to prove that Hamdan helped plot al-Qaeda attacks on civilians or other targets.
Defence lawyers said they would appeal the verdict after questioning whether Hamdan's role as a driver qualifies as a war crime. They also allege dubious evidence was presented in the case from coercive interrogations.
"Is material support a war crime? The defence believes it is not," one of Hamdan's lawyers, Michael Berrigan, told reporters after the verdict.
"That issue will go forward on appeal."
Berrigan said it was a "travesty" that Hamdan was convicted under a law adopted in 2006, which established the military commissions, long after he was captured in 2001 in Afghanistan.
But a legal expert and former prosecutor, Jonathan Drimmer, said it was "a good outcome for the government in arguing that this was a fair trial".
The case indicated the jury was independent, was able to weigh the evidence and to make a decision based on evidence, "three of the critical hallmarks of a fair trial", said Drimmer, a law professor at Georgetown University.
"The fourth factor, the ability of the defence to mount a full defence, is where the criticism of the process likely will be levied," he said, citing the hearsay and secret evidence allowed in the special tribunals.
The Yemeni national with a fourth-grade education has been at the centre of intense legal battles in which the Bush administration has fought to prosecute terror suspects outside regular courts and to hold others indefinitely without charge.
The military commissions have faced repeated legal challenges and Hamdan's appeal could have far-reaching consequences.