The note, which also threatened the mayor's brother Councilor Doug Ford and Deputy Mayor Norm Kelly, warned that Ford had "24 hours to vacate" otherwise "City Hall will blow."
"If you walk, everything is fine but if you ignore this warning, City Hall will disappear from the map," according to the note.
This is the threat Fords say they received. "Bombs already set up." pic.twitter.com/2lNlFtMnvq— Natalie Alcoba (@nataliealcoba) August 11, 2014
Bomb-sniffing dogs were brought in and no evidence of explosives was found.
"It mentioned my name, that if I don't resign immediately that the place is going to get blown up," Mayor Ford told reporters. "This is very, very serious. I don't want people to panic, but we're aware of this and police are working on this as fast as possible to find out who sent it."
Ford plans on seeking a second term as mayor in an October election.
Toronto Councilor Paula Fletcher said she hoped Ford confirmed the threat was a fake before alerting the public.
"I hope he's followed what I would call the good rules, or good common sense. And since we're still here in the building, I'm going to assume it was just an angry letter from a disturbed individual," she told CBC News.
Toronto police continue to investigate the incident.