Attorney Jo Ann Oliver described Mr Ressam as "not a smooth operator."
Ahmed Ressam, 33, stood with his eyes downcast as a clerk read out the verdict at the Los Angeles district court - guilty on nine criminal charges including conspiracy to commit terrorism.
Mr Ressam - who pleaded not guilty - could face up to 130 years in prison.
He was arrested on 14 December 1999 in Washington state, while trying to enter the United States from Canada with enough explosives to make at least four high-powered bombs, hidden in a rental car.
Pleas rejected
The jury, which took less than two days to reach a decision, rejected Mr Ressam's claims that he was an unwitting participant in the planned bombings.
Authorities said it was unclear whether the bombs were intended for four different targets or whether they were intended to be set off in rapid succession in or around the same target.
Millennium celebrations in Seattle were cancelled as a precaution after police found that Mr Ressam had reserved a motel room near the city's landmark Space Needle.
'Inconclusive' evidence
Defence lawyers accepted the explosives and false identification charges, but said evidence linking him to a wider conspiracy was inconclusive and subject to "innuendo, speculation and sensationalism."
Attorney Jo Ann Oliver described Mr Ressam as "not a smooth operator."
She went on to suggest that Mr Ressam's actions were masterminded by an accomplice, Abdelmajid Dahoumane, who was recently arrested in Algeria.
On Friday, Mr Ressam was given a five-year sentence by a French court, where he had been tried in absentia on separate charges of belonging to a network of Islamic militant groups.
US investigators believe Mr Ressam attended militant training camps in Afghanistan, and may have links to Osama Bin Laden, the alleged mastermind of two bombings of US embassies in Africa in 1998.
Despite these allegations, Judge John Coughenour had banned prosecutors from mentioning Mr Bin Laden's name during the US trial.
Mr Ressam is due to be sentenced on 28 June in Seattle, from where Judge Coughenour moved the trial due to publicity and security concerns.