In his final report, the inspector general for Iraq reconstruction estimated the US wasted at least $8bn.
Stuart Bowen put the "limited positive effects" down to corruption, poor security and insufficient consultation with Iraqi authorities.
The eight-year war in Iraq cost the US about $800bn and nearly 5,000 lives.
In Learning from Iraq, a final report to the US Congress before his office ceases operations, Mr Bowen says the failures in Iraq offer lessons for reconstruction efforts elsewhere.
The US plans to withdraw combat troops from Afghanistan by the end of 2014.
The report was based on hundreds of audits and inspections across Iraq, as well as dozens of interviews with Iraqi and American officials and politicians.
"The general belief across each group is that the relief and reconstruction programme should have accomplished more, that too much was wasted, and that the lessons derived from the Iraq reconstruction experience should drive improvements to the US approach to stabilisation and reconstruction operations," Mr Bowen reported.
Iraqi Prime Minister Nouri al-Maliki told Mr Bowen there had been "misspending" of US money.
The US-funded rebuilding programme "could have brought great change in Iraq" but fell short, he said.
Osama al-Nujaifi, speaker of the Iraqi parliament and the country's top Sunni Muslim official, said rebuilding projects "had unfavourable outcomes in general".
And Kurdish official Qubad Talabani told auditors: "You think if you throw money at a problem, you can fix it. It was just not strategic thinking."
Senator Bob Corker, the top Republican on the Senate's foreign relations committee, said the report's findings were "appalling" and that lessons must be learnt to avoid repeating mistakes in Afghanistan.
The US has spent about $90bn on reconstruction in Afghanistan over the course of a 12-year military campaign.
"Learn"...... are you kiddingme? Our politicians are going to "learn" from their waste?.... That went into their pockets and the pockets of their contractor friends!!!
Wonder what the blabbering idiots have to say at TFF?
Sure thing. The real question is: How long before it happens again? And where?