The case marked the first time that anyone charged with providing support to ISIS has taken the case to a jury.
"In the first jury trial in the country involving a homeland attack committed in the name of ISIL, Abdul Kareem was convicted of conspiring to provide material support to the foreign terrorist organization and other federal offenses," Assistant Attorney General for National Security John P. Carlin said in a statement.
Kareem, 44, was also convicted by a federal jury in Arizona of conspiracy to transport firearms and ammunition in interstate commerce with the intent to commit murder and aggravated assault, transporting firearms and ammunition in interstate commerce with the intent to commit murder and aggravated assault, making false statements to the FBI, and being a felon in possession of a firearm.
His co-conspirators were killed in a gunbattle with police outside their intended target in Garland, Texas:
An event featuring cartoons of the Prophet Muhammad deemed offensive to Muslims.
The attackers, Elton Simpson and Nadir Soofi, were wearing body armor and firing assault rifles when they were killed.
Simpson had been in contact with known jihadists before the attack, according to law enforcement sources.
"Today's guilty verdict, in one of the country's first trials involving terrorist acts committed in the United States by ISIL supporters, demonstrates our office's deep commitment to combating terrorism," Acting U.S. Attorney Elizabeth Strange of Arizona said.
Kareem, also known as Decarus Thomas, is scheduled to be sentenced June 27 and could face a sentence of at least 45 years in prison, prosecutors said.
Federal prosecutors say Kareem, Simpson and Soofi watched extremist videos together before the May 3, 2015, attack, including some depicting torture and executions.
The men allegedly conspired to commit jihadist attacks and also considered multiple potential targets, including military bases, shopping malls and the site of the 2015 Super Bowl in Glendale, Arizona, according to a December 2015 indictment.
In preparation for the attack in Garland, Texas, prosecutors said, Kareem arranged shooting trips in the remote desert outside Phoenix, where he trained Simpson and Soofi how to shoot assault rifles. He then encouraged the men to carry out the attack, prosecutors said.
Kareem feigned having been struck by a car and attempted to make an insurance claim based on his injuries to raise money for attacks, according to the indictment against him.
In addition to providing arms to Simpson and Soofi, Kareem hosted the two men and others at his home to discuss the attack on the cartoon contest, the court documents said.