McDonough is a third-generation synth living in Diamond City. He impersonates the brother of John McDonough, his human counterpart that grew up in Diamond City. As part of his infiltration, he had his eyes set on the office of the mayor. He was elected to the position by fear-mongering against the city's ghoul population, and promising to expel them from the city as part of his "Mankind for McDonough" platform. His anti-ghoul campaign strategy won him the election and the city turned on the ghoul population, alienating McDonough from his brother John, who tried to save as many ghouls as he could from dying out in the ruins.
Since his election, McDonough has been a popular mayor among merchants due to his emphasis on keeping Diamond City prosperous and safe, but something of a conflicted figure among the Lower Field population due to the circumstances of his election and his refusal to investigate the spate of kidnappings. He has a particular vendetta with Piper Wright, due to the nature of her newspaper, Publick Occurrences, and has temporarily banned her from the city. Specifically, Piper has published an article (truthfully) accusing McDonough of being a synth.
In Sheep's Clothing: McDonough is confronted in his office after it's revealed that he is a synth and is holding Geneva hostage. The Sole Survivor can convince him with a medium Speech check to release his hostage to prevent her from being harmed.
McDonough has shown a desire to join the SRB of the Institute as a courser; however, terminals show that McDonough's synth body would be incompatible with the lifestyle of a courser, and state that if he should ever be outed from Diamond City as a synth, he would be abandoned with no Institute support and an estimated survival time of two weeks.
If the player character enters his room they can see two mannequins in a corner, one is dressed in a sweater vest and one is dressed in a red coat. This is possibly a reference to him and John Hancock.
While listening to John Hancock describe his childhood with McDonough, he starts several sentences with "Guy". This was originally thought to be Mayor McDonough's first name, however, similar sentences start that way in later dialogue.
If you listen to the Mayor's second response to Piper being in the room, when he says NOT, his voice will sound slightly robotic with a louder tune. This is an early give away that he is a synth.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
Piper's interactions with Mayor McDonough seem to be inspired by Bob Woodward's and Carl Bernstein's reporting of the Watergate scandal which eventually led to the resignation of President Nixon. This is further lampshaded by Mayor McDonough's "I Am Not a Synth" speech.
End of information based on unverified behind the scenes information.