A judge rejected the councils' claim that the mayor's plans to close 10 fire stations were legally flawed for a number of reasons, including a failure to take into account the reality of increased fire risks in inner London.
Campaigners say the cuts, which involve the loss of more than 500 jobs and the axing of 14 engines, would be "reckless, wrong and will seriously endanger lives".
But Mr Justice Foskett ruled the process by which the closure decision was reached was lawful.
He said: "I appreciate that the outcome will come as a disappointment to a number of people who had hoped to see the proposed changes to the provision of fire services in their area set aside."
More top news
An investigation has begun after a teenager was stabbed in south London.
A dramatic blaze which tore through a building site in London's West End, prompting fears of an explosion, has been brought under control.
Childhood pursuits such as climbing trees or flying kites in public parks could soon attract hefty fines in one London borough.