The artwork with the word 'London - Everyone welcome' - sprawled across it with paintings of the globe incorporated in the lettering was launched on Monday (March 27).
Deputy Mayor for Culture and Creative Industries, Justine Simons and Heathrow Chairman Lord Deighton were at the airport for the unveiling.
Over the next six weeks, 10million passengers are expected to travel through the hub airport, where the iconic David Shirgley art piece will be featured.
The estimated £200,000 advertising space has been donated by the airport to the campaign to show London is united and ready to welcome the world, for business and leisure.
Mr Khan hailed the display which has just gone on show.
He said: “I’m delighted that Heathrow has joined with us to show millions of passengers from across the world that London Is Open.
"We’ve been spreading our message of welcome ever since the referendum, and now more than ever it is vital that we unite behind that.
"London remains the best place in the world for business and tourism, and it will stay that way.”
Heathrow Chief Executive John Holland-Kaye added: “Heathrow's global connections help make London a world city.
"Our support for #LondonIsOpen will show millions of international passengers that London is the best place in the world to visit, to do business and to live. ”
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