An 83-year-old man becomes the oldest inmate to be put to death since capital punishment was reinstated in the US in the 1970s.
Walter Moody was convicted of killing judge Robert Vance after sending him a mail bomb in 1989.
He was pronounced dead at 8.42pm local time on Thursday following an injection at the Alabama prison at Atmore.
He gave no final statement and did not respond when an official asked if he had any last words.
At his 1996 trial, prosecutors described Moody as a meticulous coward who committed murder by mail because of his obsession with getting revenge on the legal system, and then committed more package bombings to make it look like the Ku Klux Klan was behind the judge's murder.
His lawyers did not raise his age in legal filings, but they unsuccessfully argued in a clemency petition to Alabama's governor that his age and health would complicate the lethal injection procedure.
The US Supreme Court temporarily stayed execution plans on Thursday evening to consider Moody's late appeals, but later lifted the stay without comment, allowing the execution plans to go forward.
Moody was first convicted in 1991 in federal court and sentenced to seven life terms plus 400 years. He was later convicted in state court in 1996 and sentenced to death for Mr Vance's murder.
In last-hour appeals, Moody's lawyers had asked the US Supreme Court to stay his execution in order to review whether his federal sentence, which was handed down first, could be interrupted.
They also argued that the aggravating factors used to impose a death sentence were improper. The high court had no comment on those last-minute appeals on Thursday.
Mr Vance's son, Robert Vance Jr, now a circuit judge in Jefferson County and Democratic candidate for chief justice in Alabama, said it is important that people remember how his father lived, not just how he died.
"He was a great judge, a great lawyer before that, and a great father," he said earlier as the execution loomed.
Friends said the senior Vance quietly fought for the rights of underprivileged as both a jurist and a politician.
Moody had always maintained his innocence.
In recent weeks, Moody had sent a letter to the younger Vance claiming he was the innocent victim of a government conspiracy. "Had my Dad been murdered, I would want to know who had done it," Moody wrote. The younger Vance said he put the letter in the bin.
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The younger Vance made no plans to witness the execution. He said he had to make peace with his father's death, but said he has no doubt that Moody is guilty.
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