Despite going back to being a major character in the story’s 1920’s setting the series still name him as Mayhew.
Mayhew is given a wife Alice and a son that died in World War I. He lives in a small house and is clearly ill coughing badly, not helped by the smog and so hopes to do something for his wife with his job. Upon meeting Leonard Vole Mayhew decides to help him as he is like their son, he does go to see Romaine at the theatre and is engrossed by her singing, he tries to get an alibi from her but there isn't a chance a loving wife’s statement could let Leonard off. At the trial Romaine statement followed by the others is making Vole’s chances of acquitted weak, Sir Charles Carter unable to find anything to damage the statements apart from Janet McIntyre having a lesbian crush on Emily French.
However Mayhew gets the letters proving Romaine’s perjury from Christine Moffat who was the lead singer who was disfigured by Romaine he vows to get Romaine swearing on his late son. The letters work in showing Romaine deception and the Prosecution falls apart, after Leonard is acquitted he says he's disappointed to Romaine who hisses at him from beyond the bars, then Janet clearly upset says he is as guilty as Leonard with blood on his hands and before he could return Leonard’s thanks, he blacks out. He wakes up in hospital paid by Vole his illness was Bronchitis he is given a promotion, during the selling of Emily French’s house, he finds the frozen remains of Mimi French’s cat which tells him that Janet McIntyre was the killer since, she had blood on her sleeves and the resulting trial has her hanged after she fell apart in the trial.
Mayhew and Alice go on holiday in the South of France, as he enjoys the view he sees Leonard who has married, he goes to congrats Vole with a bottle of Champagne but he discovers that the bride is Romaine with a dyed-bob cut. Mayhew then realizes that Romaine faked the letters and that Leonard really did kill French. He leaves calling them monsters but they mock him as his generation let the young died or damaged and he declared that McIntyre was guilty.
Later, back in his room, John asks Alice if she loves him because she never says it. Breaking down, he pleads with her to say it, stating that everything he has done has been for her. Alice initially refuses to answer, but eventually confesses that she doesn't love him anymore. Alice admits that she can never forgive him for letting their underage and overly eager son fake his age, in order to accompany him to the trenches. Stating that the boy should never have gone, Alice lets Mayhew know that while she will support him, he will never have her affection again. When Mayhew once again says that everything he has done has been fo her, she calously states it was all in vain and that if he wants love he can pay for a mistress.
Although Alice is pleased to have finally gotten this off of her chest and is happy that she doesn't need to continue lying anymore, for John this revelation is the final straw. Later that night, as Leonard and Romaine drive off into the night, they unknowingly pass John Mayhew, who has gone down to the beach by himself. John stares out across the horizon, a completely crushed and devestated man. In just one day, his life has been turned upside down. He is tormented by the knowledge that his wife no longer loves him and the guilt that his son died due to own selfish actions. Worst of all though, his career, the only thing he has left is essentially ruined. Not only did he help a guilty man escape his rightful punishment, but he also sent a completely innocent woman to her death in the process.
Overcome with grief, despair and remorse, the completely broken John decides that he has nothing left to live for any longer. In a trance like state, Mayhew calmly takes off his glassess and walks slowly into the sea to die. As the tide rises, John Mayhew simply sinks beneath the surface, never to be seen again.
He was played by Toby Jones.