The Prime Minister had always been seen as a sceptic of the high speed line and the impact it would have on communities, many of them traditional Conservative areas.
But, in a visit to Birmingham's Curzon Street, which will become a High Speed 2 station, he insisted he was an enthusiastic advocate, whilst also accepting that the project needed to improve its image among members of the public.
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HS2 was approved on Tuesday (11) after facing years of delays and running over budget by more than £80 billion in a decade.
The first phase will run from London to Birmingham and Crewe before later pushing further north to Manchester and Leeds.
Mr Johnson says the high-speed rail line will become a transport "spine" of the country and will also help to improve transport services throughout different regions, including the West Midlands.
He said: "I have never been opposed to HS2. My constituency gets affected in a big way by HS2.
"I have always had real doubts about the way HS2 had handled members of the public, I have always had anxieties about the costs, but I always thought high-speed rail, per se, in itself, was a great thing.
"It was a very tough decision to look at all the issues. But there was no doubt in my mind that, for the future of this country, we have got to do HS2.
"The reason is, yes, we are doing some fantastic stuff with local transport improvements and we have made a huge investment in roads, a big investment in local rail, a big investment in buses, bicycles, you name it.
"All that local stuff, and regional stuff, will be greatly intensified, and the benefits will be greatly increased, if you can address the spine of the country.
"We want to rebuild the spine of the country, of the transport and network anatomy of the UK, and drive up lots local improvements as well."
Mr Johnson also praised West Midlands Mayor Andy Street's plans for new a new metro system across the region.
Mr Street has unveiled plans for eight new Metro lines and 21 new rail stations, under a £15billion scheme.
Mr Johnson continued: "I'm a big supporter of Mayor Andy Street's vision of better connectivity in the West Midlands, and the things he wants to do with rail and light rail, to create a kind of metro system."
But while Mr Johnson was praising of the Metro plan, he said the Government will be keeping a close eye on West Midlands Trains - a franchise which runs rail services in the West Midlands. The company has been performing poorly and it was recently ordered by the Government to invest £20m in its services in a bid to improve things.
"There is no question when franchises are failing in their duty, their contractual obligations, we will hold their feet to the fire, that is what Grant Shapps (transport secretary) is doing," said Mr Johnson.