Gardner, 49, is only the third person in 33 years to die by firing squad in the United States.
He was killed at 12:20 a.m. MST (2:20 a.m. ET), a Corrections Department spokesman said.
A hood was placed over Gardner's head and a paper target pinned to his chest. He was heavily restrained as a five-person firing squad took aim at the target and shot him, witnesses said.
Journalists who witnessed the shooting said it happend quickly. One reporter said she noticed that Gardner moved after he was shot.
"It was over pretty quickly," said Cheryl Worsley, a local radio reporter. "It was cleaner than I expected. It was fast. But he moved. He moved a little bit, and to some degree that bothers me."
Outside the prison in Draper Utah, there was a candlelight vigil attended by members of Gardner's family.
"He did things bad in the past, but now he realizes what he did and he changed the last couple of years," said his granddaughter Darian Gardner, breaking into tears. "I got to know him the last couple of years. It sucks when you have to say goodbye to someone that is like your best friend."
The execution came after Gardner's last-ditch efforts to save himself had failed.
Video: Death row inmate executed
Video: Death by firing squad
RELATED TOPICS
* Ronnie Lee Gardner
* Capital Punishment
* U.S. Supreme Court
The U.S. Supreme Court rejected a request Thursday to temporarily stay the execution. Utah Gov. Gary Herbert rejected two appeals by Gardner's attorneys to stop the execution, saying he had every chance to present his case.
Gardner, 49, was convicted for the shooting death of attorney Michael Burdell during a botched escape attempt from custody in 1985 at a Salt Lake City, Utah, courthouse.
Among the claims Gardner raised in his appeals is that he has been a death-row inmate for too long.
"He asserts that executing him now, after nearly 25 years on death-row in Utah, so lacks retributive or deterrent value that it violates the Eighth Amendment," Andrew Parnes, Gardner's lawyer, told the high court.
A federal judge late Tuesday declined to block the execution, after Gardner claimed the procedures related to a two-day commutation hearing held by the state Board of Pardons and Parole last week violated his civil rights. The parole board Monday declined to commute Gardner's sentence to life in prison, and the Utah Supreme Court on Tuesday also denied his request for a stay.
Gardner was the third person to die by rifle fire, all in Utah, since the Supreme Court restored the death penalty in 1976. He had a long history of escapes and was slipped a gun before he fatally shot Burdell on April 2, 1985. He was at the courthouse for a pre-trial hearing in the 1984 slaying of Melvyn Otterstrom, who was killed at the Salt Lake City bar where he was working to earn extra money.
Corrections officials announced Gardner had consumed his last meal Tuesday night at 6:30 p.m., and chose to fast prior to the anticipated execution. His dinner included steak, lobster, 7-Up, apple pie and vanilla ice cream.