The London Mayor will replace Daniel Moylan, who replaced the popular Baroness Margaret Ford only three months ago.
"Securing the future of the Olympic and Paralympic legacy and building on the regeneration of East London is a matter of huge importance to me personally," said Johnson.
"It is vital that I continue to be at the forefront of the decision-making, driving forward the huge task of delivery.
"The chairmanship of the LLDC will enable me to do just that, building on what has already been achieved."
The LLDC took over from the Olympic Park Legacy Company following a proposal from Johnson in January.
And top of the agenda for Johnson will be finally securing the future of the Olympic Stadium, for which there are four bidders.
They are Premier League West Ham United, League One side Leyton Orient, Intelligent Transport Services in association with Formula One, and UCFB College of Football Business.
An original deal for West Ham to lease the £486m stadium was scrapped last year amid legal wrangling.
The London Legacy Development Corporation (LLDC) said bids will be assessed before any negotiations begin.
Joint use by two East London football clubs with a running trcak retained looks to have the edge from a neutral stand point.Plans for a grand prix circuit were tabled by Intelligent Transport Services, in conjunction with F1, in what was to many a surprise move
F1 chief Bernie Ecclestone said "If they were to get permission to do it, then we would be more than happy to do something with them. But we have nothing to do with putting in a bid."
It is understood Orient's proposal is based on a ground share with West Ham, although the Premier League club has insisted it is not considering that arrangement.
The £486m stadium will be reduced from its 80,000 capacity to 60,000 seats now the Games are over.