A panel of at least five military officers will later recommend a sentence.
The 31-year-old former farmhand from Adelaide pleaded guilty on Monday to providing material support for terrorism.
He is the first person to be convicted in revised military tribunals created by US congress.
The Supreme Court had struck down an earlier version of the trbunals that George Bush, the president, authorised to try foreign captives on terrorism charges.
As part of the deal, Hicks also agreed to withdraw his previous claims that he was abused by the US military.
Hicks acknowledged that he trained with al-Qaeda in Afghanistan, fought with its forces against US allies in Afghanistan in late 2001 for two hours, and then sold his gun to raise cab fare and tried to flee to Pakistan.
He denied having any advance knowledge of the September 11 attacks.
Under the plea agreement Hicks explained why he was conducting surveillance of the US Embassy in Kabul in August 2001, more than a decade after it closed.
Hicks said that was a training exercise for one of four al-Qaeda courses he took in Afghanistan to learn urban and mountain warfare techniques and the use of disguises.
The Australian also admitted to hearing a talk by Osama bin Laden in Arabic during his time at a training camp.
He said he told the al-Qaeda leader that there was a lack of materials in English.