Colleen LaRose had faced a potential life term. But Judge Petrese Tucker accepted a government request to reduce the sentence, because of her extensive cooperation with investigators.
Prosecutors still asked for decades in prison, fearing she remains dangerous.
Larose, 50, of Pennsburg, Pa., told the judge she once had thought about jihad from morning to night, saying she was "in a trance."
"I don't want to be into jihad no more," she said.
She was sentenced to 10 years in prison plus five years of supervised release. She could be out in a little over four years, given the more than four years she has already served and the potential for time off for good behavior.
Prosecutors depicted LaRose as a "lonely and isolated" woman who sought excitement by joining the jihadist cause. She was flattered to get an assignment to kill a foe of Islam.
U.S. investigators say she participated in a 2009 conspiracy to target artist Lars Vilks over his series of drawings depicting the Muslim prophet Muhammad as a dog. Muslim extremists in Iraq had offered a $100,000 reward for anyone who killed Vilks, who was never attacked.
The Justice Department said Ali Charaf Damache, who was living in Ireland, recruited LaRose and another U.S. woman via jihadist websites.
Damache married the other woman, Jamie Paulin-Ramirez, in a Muslim ceremony on the day she arrived in Ireland from Colorado that same month.
LaRose left the terror cell in Ireland after about six weeks not because she thought better of the murder plot, but because she "grew frustrated because her co-conspirators were not ready for action," Assistant U.S. Attorney Jennifer Arbittier Williams said Monday.
Others need to know, she said, that "if you plot to kill someone, you are going to receive decades behind bars - decades - even if you cooperate," Williams said.
Public defender Mark Wilson, though, said LaRose has come to understand the true, peaceful tenets of Islam and said "there's virtually no chance that she would ever be involved in violent jihad ever again."
Judge Tucker said she had no doubt LaRose, who stalked Vilks online, would have killed him had she had the chance.
"The fact that out of boredom, or out of being housebound, she took to the computer and communicated with the people she communicated with, and hatched this mission, is just unbelievable," Tucker said.
Vilks told The Associated Press that he understands tough sentences can act as deterrents but said he felt LaRose's term was too harsh.
"To lock her up for so many years seems like overkill to me," Vilks said. "This is a person who has been through a lot of difficulties in her life and needs mental care more than anything else."
Vilks said he is still under threat but that he has around-the-clock protection that makes him feel safe.
LaRose returned to Philadelphia in 2009 to surrender, becoming one of the few women ever charged in the U.S. with terrorist activities. Her arrest was kept secret and the indictment unsealed only after Paulin-Ramirez and the six others were rounded up in Ireland months later.
Paulin-Ramirez and another co-defendant, Maryland teen Mohammad Hassan Khalid, also are scheduled to be sentenced this week in Philadelphia.
Associated Press writer Malin Rising in Stockholm contributed to this report.