The endorsement from the Uniformed Firefighters Association comes weeks after Thompson picked up the backing of a coalition of uniformed public safety unions. The UFA also supported Thompson during his failed 2009 bid against Mayor Michael Bloomberg, who cannot seek re-election this year.
"We live in a post-9/11 world and we need someone, a leader," said union president Steve Cassidy. "The city has nothing unless it has public safety. And Bill Thompson recognizes that."
The UFA has about 8,200 active members who live in the city's five boroughs.
The city's labor force had hoped to unify behind one candidate in the Sept. 10 primary, but has splintered among several mayoral hopefuls. Thompson, City Council Speaker Christine Quinn, Public Advocate Bill de Blasio and city Comptroller John Liu have each received endorsements from major unions.
Polls show Thompson, Quinn and former congressman Anthony Weiner are the top Democratic contenders.
In recent years, the Uniformed Firefighters Association has clashed with Bloomberg over his budget proposal to shutter 20 fire companies. Each time, the City Council restored funding to save the companies.
On Tuesday, Cassidy acknowledged Quinn's role in saving those FDNY units but said it was not enough to win his union's support. He then touted Thompson's "temperament," which appeared to be a subtle swipe at Quinn's much-discussed temper. Cassidy said the UFA, which backed Weiner during his 2005 mayoral run, never considered supporting him this time.
The Quinn campaign responded by sending a statement from Al Hagan, president of the Uniformed Fire Officers Association, which endorsed Quinn earlier this year. Hagan said Quinn "led the fight" to save the fire companies.
A truck carrying a billboard bearing Thompson's face and the UFA logo provided the backdrop for Tuesday's press conference, which was held in front of a Lower East Side firehouse. Union officials pledged to deploy the truck daily until the September primary and promised additional expenditures on Thompson's behalf, though they did not specify an amount.
The general election is Nov. 5.