Mr Johnson had planned to make his announcement in his Telegraph column, but was instead forced to make a day-early statement outside his North London home on Sunday (February 21).
He told the crowds: "I've made up my mind.
"It's not about whether you love Europe or not. I love Brussels. I used to live in Brussels - fantastic city, wonderful place, and I love European culture and civilisation.
"But there should be no confusion between the wonders of Europe and holidays in Europe and fantastic food and friendships with a political project that has basically been going on now for decades.
"I now think [it] is in real danger of getting out of proper democratic control."
It follows weeks of speculation over whether Mr Johnson would back the 'In' or 'Out' camps ahead of the June 23 poll .
The Mayor himself told friends last week he was “veering all over the place like a shopping trolley”.
Tim Farron, Lib Dem leader, said: “This is a deeply cynical move from a deeply ambitious politician who is using an in-out referendum as a back door to Number 10.
“It is a selfish move to put personal ambition before the jobs, security and prosperity of every Londoner.”
Mr Johnson is seen as potentially key in the referendum campaign as one of the few politicians with genuine popular appeal.
A recent poll by Ipsos Mori found a third of voters said the London Mayor's view could influence their vote.
He said he respected the PM but added: "I don't think anybody could realistically claim that this is fundamental reform of the EU or of Britain's relationships with the EU.
"It's my view that of 30 years of writing about this we have a chance actually to do something".
Earlier on Sunday, the Prime Minister was reduced to begging the London Mayor live on TV not to join forces with 'Out' campaigners like UKIP chief Nigel Farage George Galloway.
Mr Cameron told the BBC's Andrew Marr Show: "I would say to Boris what I say to everybody else - we will be safer, we will be stronger, we will be better off inside the EU.
"I think the prospect of linking arms with Nigel Farage and George Galloway and taking a leap into the dark is the wrong step for our country.
"And if Boris and if others really care about being able to get things done in our world, then the EU is one of the ways in which we get them done."
Bur Mr Johnson ignored the advice and will now be a leading figurehead for the 'Out' campaign along with six rebel Tory Cabinet Ministers including Work and Pensions Secretary Iain Duncan Smith and Justice Secretary Michael Gove.