As mayor, Boris Johnson continued to push his "pet project" although it talked key funding and consents, and his actions ultimately contributed to its failure, the cross-party report found.
The tree-lined pedestrian bridge would have spanned the Thames between Temple Station and the South Bank, and was intended to open in 2018.
First proposed by actress Joanna Lumley twenty years ago, the bridge was championed by Mr Johnson during his second term as Mayor.
The scheme cost the taxpayer almost £43 million – but it was never built.
In 2017 current Mayor Sadiq Khan withdrew City Hall's support for the project.
Now the London Assembly has said the Prime Minister, the Garden Bridge Trust and Transport for London (TfL) are all responsible for the bridge's failure.
The trust took "increasingly reckless" financial decisions which jeopardised its plans, the report found.
Crucially, it signed a construction contract without fully securing the land to build the bridge, or the necessary planning consents.
But despite this, Mr Johnson removed obstacles to ensure the project went ahead, according to the Assembly.
The then-Mayor ordered TfL to set aside £30 million to fund the bridge, matching the £30 million to be provided by the Department for Transport.
But although this was originally dependent on the trust sourcing a further £115 million of private funding, Mr Johnson "significantly softened" this condition.
Later, the trust only had to provide a financial plan – this was a "pivotal change" in which the Prime Minister is personally implicated, according to the Assembly.
TfL is also partly responsible for the failed project, and should have made the risks clearer to the Mayor, the report claims.
Tom Copley, chairman of the London Assembly's Garden Bridge Working Group, said the bridge was "an appalling waste of public money".
He said: "Across all key decision-making organisations, there were consistent failures to adequately, effectively and transparently deal with the escalating risks of the project.
"It is unacceptable that these risks were being downplayed by Transport for London, in order to fulfil the former Mayor, Boris Johnson's, pet project."
He added: "Squandering taxpayers' money on doomed large-scale infrastructure projects is not acceptable and I hope this report goes some way to ensuring that those responsible change their high-handed attitudes and learn from their stupid mistakes."
But Downing Street said the current Mayor Sadiq Khan is responsible for scrapping the bridge, and accused the Assembly of trying to score political points.
Paul Scully, deputy chairman of the Conservative Party, said: "Sadiq Khan's flip flopping on this scheme led to its demise – leaving Londoners paying the price.
"Labour should stop playing politics and start focusing on making London's streets safer."
A TfL spokesperson said: "We welcome the report by the London Assembly Garden Bridge Working Group on the learnings of the Garden Bridge project.
"We will review the report and respond to the recommendations for TfL in due course."
The Garden Bridge Trust has now ceased operating, and submitted a statutory declaration of solvency as part of its shutdown process earlier this year.
Trustees refused to appear at a meeting of the working group, with their chairs being left empty during a City Hall hearing.