The event, which marked the completion of the external structure of the building, saw Johnson secure the final beam on the hotel's penthouse floor.
When it opens early next year the £350 million, 16-storey, glass-panelled venue will feature more than 1,000 guest rooms and 2,700sqm of event space.
It will employ approximately 800 people and will boast a selection of new restaurants, a stylish bar and lounge and a luxury spa.
"A sparkling new hotel is just what the South Bank needs," said Johnson. "I am sure it will act as a major springboard for the regeneration plans for Waterloo Station and the surrounding area, on top of creating hundreds of jobs and boosting the local economy."
"What's more, the huge investment being made today demonstrates great confidence in the future prosperity of our great city."
Park Plaza Hotels already operate four four-star deluxe and one five-star hotel in London.
The new Park Plaza Westminster Bridge is located on the former site of the derelict Greater London Council (GLC) building, which was widely regarded as one of the ugliest buildings in London.
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