In a Louisiana assisted-living home in 1999, Paul Edgecomb begins to cry while watching the film Top Hat. His companion Elaine becomes concerned, and Paul explains to her that the film reminded him of the events of 1935, which took place when he was a prison officer, in charge of death row, what they refer to as the "Green Mile".
In 1935, Paul supervises officers Brutus Howell, Dean Stanton, Harry Terwilliger, and Percy Wetmore at Cold Mountain Penitentiary. Paul, who suffers from a severe bladder infection, receives John Coffey, a physically imposing but mentally challenged black man, into his custody. John had been sentenced to death after being convicted of raping and murdering two white girls. One of the other inmates is a Native-American named Arlen Bitterbuck, who is charged with murder and is the first to be executed. Percy demonstrates a severe sadistic streak, but, as the nephew of Louisiana's First Lady, he cannot be held to account; he is particularly abusive towards inmate Eduard Delacroix, breaking his fingers, killing his pet mouse, Mr. Jingles, and repeatedly calling him "faggot". In exchange for resigning from the penitentiary and accepting a job at an insane asylum, Percy is allowed by Paul to oversee Del's execution; at the execution, Percy deliberately avoids soaking the sponge used to conduct electricity to Del's head, leading to Del suffering a gruesome and agonizing death with Paul and the others in attendance.
John begins to demonstrate supernatural powers; he cures Paul's bladder infection, resurrects Mr. Jingles, and heals Melinda Moores, wife of the prison's chief warden, of a brain tumor. This last affliction he releases into Percy, who under its influence shoots prisoner William Wharton to death. Wharton had from the moment of his arrival been a troublemaker; he assaulted the guards as he was being escorted into the block, made mischief on two occasions that later caused Paul to order him restrained in the block's padded cell, groped Percy, racially insulted John, and revealed psychically to John that he is, in fact, responsible for the crime for which John was condemned. John then reveals the story psychically to Paul, and, when doing so, he also releases his supernatural energy into Paul. Percy is committed to the insane asylum.
Although distraught over the notion of being executed when innocent, John tells Paul that he wishes to die, as he views the world as a cruel place and constantly feels the cruelty of humanity like shards of glass in his head. Mentioning that he had never seen a movie before, John watches Top Hat with the other guards as a last request. John is executed that night; he asks the customary hood not be placed over his head, as he is afraid of the dark. Paul concludes his story by telling Elaine that John's was the last execution that he and Brutus supervised; they both subsequently resigned from the prison and took jobs in the juvenile system.
Elaine realizes that, since he had a grown son in 1935, Paul must be much older than he looks. Paul reveals that he is 108 years old. He is kept alive by the power that John passed on to show his psychic memories. Del's mouse, Mr. Jingles, is also still alive.
Paul is shown attending Elaine's funeral, and muses that if John's power could make a mouse live for as long as Mr. Jingles has, how much longer does he himself have left?
