The target was highly symbolic, the headquarters of the Birmingham movement where King and his followers launched waves of protests aimed at desegregating a city known then as the Johannesburg of America.
A jury of eight white and four black people convicted Thomas Blanton Jr, 62, for the attack on a black church congregation in the city of Birmingham, Alabama.
Blanton was immediately sentenced to life in prison for what is considered one of the most shocking crimes of the civil rights era.
"I guess the good Lord will settle on judgment day," said Blanton, who had denied any involvement in the bombing.
Stairwell bomb
Blanton was accused of helping other Klansmen plant the powerful bomb that went off at the Sixteenth Street Baptist Church on the morning of Sunday 15 September 1963.
More than 20 fellow churchgoers were also injured, their faces cut by shards from the stained-glass windows.
Four men were suspected of the crime; one of them, Robert "Dynamite Bob" Chambliss, was convicted in 1977 and died in prison eight years later while serving a life sentence for murder.
Prosecution blocked
Throughout the trial Blanton was portrayed as a vile segregationist who planted the bomb at the Baptist church in response to months of civil rights demonstrations led by Dr Martin Luther King.
The target was highly symbolic, the headquarters of the Birmingham movement where King and his followers launched waves of protests aimed at desegregating a city known then as the Johannesburg of America.
Blanton was incriminated by conversations with his fellow Klansmen and bombers, recorded by the FBI in 1964.
At one point he was heard to say: "I like to go shooting, I like to go fishing. I like to go bombing."
He would have been brought to trial much earlier had it not been for the opposition of the then FBI Director, J Edgar Hoover, never a friend of black civil rights activists.
The case was reopened following a 1993 meeting in Birmingham between FBI officials and black ministers.
Blanton's alleged accomplice, 71-year-old Bobby Frank Cherry had his trial postponed indefinitely as a result of failing health.