A judge has cleared him of accusations he acted improperly when Transport for London declined to place a poster on the side of a London bus which said 'Not gay! Post-gay and proud. Get over it!'
Christian charity The Core Issues Trust wanted to place the advert in response to one by gay rights group Stonewall which said "Some people are gay, get over it!"
Transport for London refused the poster amid claims it would offend the majority of Londoners. The Core Issues Trust appealed that decision but it was upheld.
The charity then took the case to The Court of Appeal and claimed the Mayor acted 'improperly' and personally imposed the ban in a bid to win votes just days before the 2012 London elections.
A judge asked the High Court to decide if Mr Johnson had took the decision based on political motivations.
Mrs Justice Lang wrote in her judgement: "I consider that his response would have been the same, whether or not he was in an election campaign.
"I found his explanation for his views on the advertisements to be convincing and I accept his statement that the forthcoming election was not the reason why he concluded that the advertisement ought not to run.
"He was equally liable to be lambasted by the gay or the Christian lobby, whichever view he took of the advertisements."
Andrea Williams, from the Christian Legal Centre who led the case, told Premier: "Here we see a court that is supressing free speech and that's what this judgement does.
"We will be testing it, we will be applying to the court of appeal, we are determined not to let this rest in the interests of free speech in this nation."
Howard Carter is the general counsel at Transport for London. He said: "As we have always maintained, and the court has ruled, it was our decision not to run the Core Issues Trust adverts and there was no improper purpose behind our decision. Our decision was justified."
Hear more from The Christian Legal Centre's Andrea Williams: