At some point, the Mayor was holding a City Hall meeting. The Joker and his henchmen crashed the meeting, claimed that the city had "a paucity of parking spots designated for clown cars," and that he had tried to park in the handicapped spot, but people told him that he wasn't sick enough. The Mayor, instantly recognizing the Joker, exclaimed in shock that someone had better call the police.
Unfortunately, the Joker, "sensing" that the Mayor looked like he was about to explode, wired him up to an explosive vest that was pattered after the colors of the American Flag. An anonymous person also gave a 911 call that related to the Joker's plans seconds before the Joker's assault. Joker then declared his intent to blow up the Mayor with various explosive chemicals in a "big, bodacious burst of patriotism." Batman then intervened (ironically right after Joker had expressed his disappointment that Batman apparently wasn't coming), and unstrapped the Mayor, while the vest was blown up by a henchmen who tried to shoot at him.
The Mayor then declared that he "had enough of this city" before he announced his resignation. Due to these circumstances, it made it all the easier for Arkham Asylum Warden Quincy Sharp to win in his campaign for Mayor, a political victory that was eventually solidified when he took falsified credit for stopping Joker's plans in a brutal riot at the Asylum, which occurred that very same night.
As Sharp's victory of becoming Mayor allowed him to implement plans at the behest of his secret collaborator and manipulator, Professor Hugo Strange, which took the shape of Arkham City, Joker's attack on the Mayor and City Hall was an easily overlooked event, but was nonetheless an extremely pivotal one towards the entire series as it served as one of the catalysts for the rest of the events that unfolded over the following years.