Mohamud was accused of leading a plot to detonate a bomb at Portland's 2010 Christmas tree-lighting ceremony. But the device he thought was a bomb was a fake, supplied by undercover FBI agents posing as members of al-Qaida.
Mohamud sat still, giving no visible reaction as Thursday's verdict was read. His attorney, Steve Sady, later said an appeal was being planned after the scheduled May 14 sentencing.
"We are disappointed with the verdict," Sady said. "We obviously though he was entrapped."
Mohamud faces up to life in prison at his sentencing.
Prosecutors had argued that Mohamud was predisposed to terrorism as early as 15 years old.
Mohamud, now 21, traded emails with an al-Qaida lieutenant later killed in a drone strike. He also told undercover agents he would pose as a college student while preparing for violent jihad.
"We are hopeful that this will bring closure and healing to all of us here in Portland," said Amanda Marshall, U.S. Attorney for Oregon. "This case has been a difficult case for the city of Portland. It's been a particularly difficult case for Mohamed Mohamud's community, for his family, for the Somali community."
Mohamud was never called to testify. Instead, the jurors saw thousands of exhibits and heard hours of testimony from friends, parents, undercover FBI agents and experts in counterterrorism, teenage brain development and the psychology of the Muslim world.
"Mr. Mohamud made a series of choices over a period of several years - choices that were leading him down a path that would have ended in violence," said Greg Fowler, who leads the FBI office in Portland. "His actions showed little regard for the rights and responsibilities that come with being an American or respect for the lives that he was prepared to take."
Assistant U.S. Attorney Ethan Knight told the jury earlier this week that the decision would be easy. Mohamud pressed a keypad button on a black Nokia cellphone and intended to kill people. Whatever else they might think about the methods of undercover agents or the government's decision to investigate a teenager, the underlying decision was Mohamud's and the motivation was hatred of the West.
"It's too early to tell about sentencing specifically," Knight said on Thursday. "We'll have to wait and see what further investigation, the presentencing report, will say about the defendant."
Sady had argued that Mohamud wasn't radicalized by online recruiters or friends with jihadist leanings, but rather by a Justice Department hungry for convictions that ignored every caution sign along the way. Undercover agents manipulated Mohamud's faith and plied him with praise and the promise of a life leading other jihadis, Sady said.
Mohamud could be ordered to serve life in prison.