Ever since his election and inauguration into office, Mayor McDonough has been head of the city council for several years. He was born in Diamond City to Patrick and Martha McDonough, and had a brother, John. In 2282, McDonough was voted into office by fear-mongering residents opposing the ghoul residents in the city. He promised to expel them from Diamond City as part of his anti-ghoul "Mankind for McDonough" platform, which ultimately won him the election. New legislation was soon passed prohibiting ghouls from city limits.
Following the activation of the ordinance, McDonough entered into a feud with his brother John over the rights of ghouls in Diamond City. John attempted to persuade him to stand down, eventually attempting to rescue the majority of the city's ghoul population from being arrested when the efforts proved unsuccessful. The feud resulted in the departure of John McDonough from Diamond City, with the eviction of the city's ghoul residents following soon after. The refugees, now banished out into the wastes, were forced to emigrate to outlying communities, however a few had joined up with John, eventually settling down in Goodneighbor.
Mayor McDonough had secretly been under the observation of Institute agents for several years. At some point, the organization had sent operatives to kidnap the human McDonough and replace him with a Generation-3 synth duplicate, unit designation M7-62. Under the guise of the mayor of Diamond City, the synth has been carrying out a mission to infiltrate the city and its inhabitants, whom are oblivious to his true nature as an Institute agent. The synthetic mayor took efforts to conceal his secret allegiance to the scientific community from his people, even though an observant journalist, Piper Wright had caught whiff of his plans and published the information in a news story, titled "The Synthetic Truth." Piper has gotten on the bad side of the mayor as a result of her sensationalizing rumors of him being a synth, which has led her to receiving a temporary ban from city limits.
McDonough has been a popular mayor among merchants and the wealthy upper stands residents, 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 lead investigations into the recent spate of kidnappings, which has sparked mass paranoia within the community.
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 one listens 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.
↑SRB terminal (Lower), Infiltrator Unit: McDonough, P1: "I'm starting to wonder if M7-62 (the Infiltrator unit "McDonough") hasn't begun to outlive its usefulness. Assuming the identity of Diamond City's mayor has provided us with invaluable intelligence over the years, but suspicions have only continued to mount. This latest incident - the publication of that newspaper article specifically calling McDonough's humanity into question - might just be the tipping point."
↑John Hancock: "But then he decides he's gonna try and get elected with his anti-Ghoul crusade - 'Mankind for McDonough.'"