Lance Cpl. Antoine D. Boykins admitted cutting suspension lines on 13 of the 22 parachutes that were to be used in the exercise Sept. 21. He said he knew someone could be killed or injured because reserve chutes fail nearly half the time they're used.
Three Marines were injured in the jump's first wave, and the exercise was canceled.
The judge, Col. Alvin Keller, deliberated about five hours before sentencing Boykins. He also cut Boykins' rank to private, stripped him of all future pay and allowances and said he would be dishonorably discharged.
The sentence must be formally approved by the commander of the 2nd Marine Expeditionary Force and will be automatically appealed, prosecutors said.
"It's clear to this court that you have a great deal of natural ability and you are lucky to have a number of people who support you," said Keller, who heard testimony from Boykins' family as well as members of his church in Baltimore.
Boykins' parents were among about 15 people from his church who attended the sentencing. During a break, the group softly sang a hymn in the courtroom.
Boykins, 21, had pleaded guilty to nine counts of reckless endangerment, four counts of aggravated assault and one count of destruction of government property. Keller initially said the maximum sentence was 31 years but later recalculated it to 21 years.
Boykins originally was charged with attempted murder, but agreed to plead guilty to the lesser counts and testify against another Marine charged in the case.
Prosecutors said he deserved the maximum sentence. "This is a case about treachery," said Lt. Col. Stuart Couch.
Couch said Boykins was angry because he had been punished and his pay was docked when he returned late from leave.
Defense lawyer Kimberly Tarver of Baltimore pleaded for a lighter sentence, and asked that Boykins be transferred to military prison at Fort Leavenworth, Kan., so he can get counseling.
Before sentencing, Boykins told Keller he "made the worst decision I could ever imagine."
"Somewhere along the way, I went left where I should have gone right," Boykins said, reading from a prepared statement. "I was wrong and I am sorry."
Keller told Boykins to heed the advice of his pastor, the Rev. Stephen Andrew Tillett, who testified he told Boykins to find other Christians in prison and stick with them, and to take college courses while he serves his sentence.
"You can still be a productive citizen in this country if you take advantage of what your pastor has recommended that you do," Keller said.
Boykins will be eligible for parole after serving one-third of his sentence.
Boykins was led away by two escorts after court adjourned. His family left without commenting.
Tillett, a chaplain in the Air National Guard, said he and others in Mount Zion United Methodist Church will welcome Boykins back when is released.
"We're going to just stay behind him, send him letters, visit him," Tillett said. "Lord willing, he'll get out sooner rather than later."
Boykins is expected to testify against Cpl. Clayton A. Chaffin, 28, from Franklin, Ohio, who is charged with 31 counts, including reckless endangerment, aggravated assault, conspiracy and drug charges. Charges against a third Marine were dismissed in April.
Prosecutors allege Chaffin encouraged Boykins' crime because both were angry at their unit.