Abdul Malik Abdul Kareem was accused of providing the guns used at the event in suburban Dallas in 2015 and hosting two friends who were Islamic State followers at his home to discuss the upcoming attack.
The 45-year-old moving company owner testified that he had no knowledge beforehand about the attack.
It's unknown whether the attack was inspired by the Islamic State or carried out in response to an order from the group.
The verdict marked the second conviction in which a person was tried in the U.S. on charges related to the terrorist group.