The Minnesota Vikings will play six-time Super Bowl champions the Pittsburgh Steelers on September 29 next year.
The NFL had already confirmed that Jacksonville Jaguars and San Francisco 49ers will play on October 27, 2013.
Mr Johnson hopes the matches will bolster London's reputation as a magnet for international sport on the back of the successful staging of the 2012 Olympics and Paralympics.
He said: "London has staged an extraordinary year of sport and I want to ensure this is a springboard for even more world class sports spectaculars in the future.
"A second NFL game headed to the capital is fantastic news for sports fans and will help to inject tens of millions of pounds into the city's economy."
The NFL has played a regular-season game at Wembley in each of the last five years but this will be the first time that two games have been hosted in London in one season.
The Jaguars have agreed to play one home game in London each year from 2013 through to 2016.
Participation in amateur American football in the UK has risen over the past few years, growing by approximately 15 per cent per year since 2007, while television audiences for NFL games have risen 154 per cent since 2006.
Capacity crowds of 80,000 are expected to attend both Wembley matches, according to Mr Johnson, whose London & Partners promotional agency has been working closely with the NFL to try to bring more games to London.
NFL commissioner Roger Goodell said: "Since we started playing regular-season games in London five years ago, we have heard very clearly from our UK fans - they want more football.
"We are excited to play two games in London and take this next step in the growth of our game. We believe that more football will lead to more fans."
London is aiming to host a packed top class international sporting calendar over the coming years.
Both the Olympic Stadium and Twickenham are on the shortlist to host the 2015 Rugby World Cup and the Olympic Stadium is set to host the 2017 World Athletics Championships when the world's finest track and field athletes return to the capital.
A two-day festival of cycling will be the first large-scale event to use the Olympic Park when it reopens in 2013.
The festival will culminate in a 100-mile race for amateurs and world class competitors starting at the Olympic Park that organisers say will be similar to the London Marathon - but for cyclists.