While a jury acquitted Tahawwur Rana last year of involvement in the 2008 Mumbai attack that killed more than 160 people, it found him guilty of helping Lashkar-e-Taiba, the militant group that took responsibility for the attack, and planned bombings in Denmark.
His sentencing was set for December.
Rana's trial featured testimony from the government's star witness — his former friend, David Coleman Headley.
Headley pleaded guilty to laying groundwork before the attack on Mumbai, and plotting to bomb a Danish newspaper that printed cartoons of Prophet Muhammad in 2005.
In affirming Rana's conviction, Illinois District Court Judge Harry Leinenweber noted the jury had been presented with contrasting pictures of him.
On the one hand, the prosecution portrayed Rana, 51, as a man who knowingly supported his lifelong friend as Headley travelled the world plotting and preparing terrorist attacks.
In contrast, the defence painted him as an ambitious businessman manipulated by a friend into unwittingly providing cover for terrorist plots.
"Each side presented evidence to support its account and identified apparent inconsistencies or gaps in the other party's argument," Leinenweber said in his ruling.
"Ultimately, the court cannot conclude that no rational jury would accept the government's version of events and find Rana guilty."
Rana, a Canadian citizen, owned First World Immigration Services in Toronto, New York and Chicago, where he had lived for years.
His trial provided a rare glimpse into the inner workings of Lashkar-e-Taiba and the alleged co-operation with Pakistan's top intelligence agency known as the ISI.
In his motion for a new trial, his lawyers argued, among other things, that he should not have been forced to defend himself against involvement in the Mumbai and Denmark plots at the same time.
The court dismissed his objection on a technicality but Leinenweber also found any prejudice to him was "minimal" in that evidence related to the India attack would inevitably have surfaced at a trial on the Danish plot.
Rana also tried to have evidence from the FBI's search of his home and business suppressed on the basis the warrant lacked probable cause.
He said the warrant included damning statements from Headley's interrogation but left out those that would have helped his case.
Either way, the judge said, there was sufficient evidence to indicate a crime had been committed.
Rana also maintained the prosecution had failed to prove he knew the nature of Headley's activities, or knowingly supported Lashkar.
Leinenweber would not bite:
"A rational jury could have concluded that Rana knowingly conspired to — and did — provide material support to the Denmark plot by furnishing Headley with business cards and supporting his business cover, and by providing Headley with logistical support for his travels and plots."
Earlier this year, an Indian court ruling paved the way for authorities there to try to extradite Rana to try him for the Mumbai attack.
— By Colin Perkel in Toronto