Judge James Brady also banned prosecutors from trying Albert Woodfox, 68, for a third time.
He has been in solitary confinement since 18 April 1972 after a prison riot that resulted in the death of a guard.
Woodfox was tried twice for the guard's death, but both convictions were later overturned. He denies all the charges.
On Monday, Judge Brady ordered the unconditional release of Woodfox and barred a third trial, saying it could not be fair.
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King, who spent 29 years in solitary confinement, described his experience to the BBC in an interview three years ago.
He said he remained strong but it was "scary" to see how others crumpled through lack of human contact.