A sculpture made from a World Trade Centre girder has been permanently installed at the Olympic Park.
The 28ft (8.5m)-tall artwork was gifted to the UK by the Port Authority of New York and New Jersey in 2010 on the condition it was permanently sited.
But less than a month after it was unveiled in Battersea Park in 2011 it was removed amid claims a permanent site could not be found.
Boris Johnson said it was a "tremendous new addition to the park".
The sculpture, by American artist Miya Ando, honours the nearly 3,000 people, including 67 Britons, who died in the terror attack.
It was commissioned by UK-based charity SINCE 9/11 and uses a girder - one of only six pieces sent around the world.
In 2011 Mr Johnson said efforts to find a permanent home for the sculpture had had proved "incredibly difficult" after they were met with opposition from "boroughs and bureaucrats".
It was left to languish in a Cambridgeshire farmyard, he said.
But at Tuesday's unveiling the mayor said "This pertinent reminder of the 9/11 atrocities is a tremendous new addition to the park that encapsulated the spirit of hope, and tolerance during our Olympic Games.
"Nearly 14 years may have passed but this prodigious art work will generate continued interest, discussion and memories in the thousands of visitors to its landmark new home."
US Ambassador to the UK, Matthew Barzun said the Queen Elizabeth Olympic Park was a "fantastic location" for the sculpture.